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  #1   Report Post  
Rick Hughes
 
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Default RCD or not to RCD ...

I am, wiring up a load of new ccts .....I have 2 CU's and arranged to
provide 3 discrimination classes - no RCD protection, 30mA RCD protection,
and 100mA protection.


The question arises on Cookers & Immersion Heaters ?

I know there is no requirement in the regs for protection, but have always
had these via protected ways in previous houses .... what is the current
'best practise' for cookers & immersion heaters.

No protection, 30mA or 100mA ?


Rick


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Lurch
 
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On Sun, 8 Aug 2004 17:53:13 +0000 (UTC), "Rick Hughes"
strung together this:

The question arises on Cookers & Immersion Heaters ?

No protection, 30mA or 100mA ?

You seem to be doing well making up your own ways of wiring your house
so if I were you I'd just toss a coin!
If I were me, which I am, I'd put the immersion and cooker on no
protection, which I did.
OOI, why do you think you need 3 classes of protection?
--

SJW
A.C.S. Ltd
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Handy Andy
 
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"Lurch" wrote in message
news
On Sun, 8 Aug 2004 17:53:13 +0000 (UTC), "Rick Hughes"
strung together this:

The question arises on Cookers & Immersion Heaters ?

No protection, 30mA or 100mA ?

You seem to be doing well making up your own ways of wiring your house
so if I were you I'd just toss a coin!


And if I were you I'd learn some manners and a degree of modesty. It's quite
clear you have a great deal to be modest about.





  #4   Report Post  
Owain
 
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"Rick Hughes" wrote
| I am, wiring up a load of new ccts .....I have 2 CU's and arranged
| to provide 3 discrimination classes - no RCD protection, 30mA RCD
| protection, and 100mA protection.

Why? If you have TT earthing then *everything* should be on 100mA and
sockets on 30mA.

| The question arises on Cookers & Immersion Heaters ?
| I know there is no requirement in the regs for protection,

There are requirements in the Regs for disconnection times of circuit
protective devices. If you cannot meet the disconnection times using circuit
protective devices then you have to use RCDs. In practice, this means using
100mA RCD on all TT installations unless you can prove by calculation and
measurement that they are not needed.

(For completeness, I will mention that if you have a split load CU with a
100mA RCD main switch and a 30mA RCD on socket circuits, the 100mA RCD
should be time-delayed to provide discrimination.)

Owain


  #5   Report Post  
tony sayer
 
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In article , Owain
writes
"Rick Hughes" wrote
| I am, wiring up a load of new ccts .....I have 2 CU's and arranged
| to provide 3 discrimination classes - no RCD protection, 30mA RCD
| protection, and 100mA protection.

Why? If you have TT earthing then *everything* should be on 100mA and
sockets on 30mA.

| The question arises on Cookers & Immersion Heaters ?
| I know there is no requirement in the regs for protection,

There are requirements in the Regs for disconnection times of circuit
protective devices. If you cannot meet the disconnection times using circuit
protective devices then you have to use RCDs. In practice, this means using
100mA RCD on all TT installations unless you can prove by calculation and
measurement that they are not needed.

(For completeness, I will mention that if you have a split load CU with a
100mA RCD main switch and a 30mA RCD on socket circuits, the 100mA RCD
should be time-delayed to provide discrimination.)

Owain



I'd much prefer everything to be on a 30 ma RCD after all a 100 ma ones
not going to be much fun to try to trip through your body

FWIW we have everything going through a 30 ma one here and no nuisance
tripping apart from the odd cooker and immersion heater element but they
show those up before their likely to fail.....
--
Tony Sayer



  #6   Report Post  
Rick Hughes
 
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"Lurch" wrote in message

You seem to be doing well making up your own ways of wiring your house
so if I were you I'd just toss a coin!


I don't understand your comment ? is it just humor that I have missed, or
do you really think I am making up my own rules ? I'm not.

If I were me, which I am, I'd put the immersion and cooker on no
protection, which I did.
OOI, why do you think you need 3 classes of protection?


No protection - Alarm, Home Automation, Fridge & Freezer
100mA - fire protection
30mA - life protection


Rick





  #7   Report Post  
Rick Hughes
 
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"Owain" wrote in message
...
"Rick Hughes" wrote
| I am, wiring up a load of new ccts .....I have 2 CU's and arranged
| to provide 3 discrimination classes - no RCD protection, 30mA RCD
| protection, and 100mA protection.

Why? If you have TT earthing then *everything* should be on 100mA and
sockets on 30mA.


Supply is PME ( TN-C-S)


  #8   Report Post  
Rick Hughes
 
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"tony sayer" wrote in message
I'd much prefer everything to be on a 30 ma RCD after all a 100 ma ones
not going to be much fun to try to trip through your body



Agree on life protection, and the 30mA RCD wil provide protection to the AC
outlets, Radial and Ring Final circuits.

The Best Practise oft talked about is not to have lighting on a 30mA trip
duwe to nusisance tripping, many on this group for example have advocated no
protection for light circuits.

Now I prefer to have some protection to prvent fire hazard, and thus have
all light ccts through a 100mA RCD which are much less prone to nuiscance
tripping.

My question is really pretty simple - do I protect Imeesrion and cooker or
have them unprotected.
AFAIK the regs do not require protection, but that doen't mean i t's best
practise.


Rick


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Handy Andy
 
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"Rick Hughes" wrote in message
...
"Lurch" wrote in message

You seem to be doing well making up your own ways of wiring your house
so if I were you I'd just toss a coin!


I don't understand your comment ? is it just humor that I have missed, or
do you really think I am making up my own rules ? I'm not.


Don't worry, he's just a silly little boy. He's probably just got himself a
little City & Guilds and now he thinks he's an expert!


  #10   Report Post  
Owain
 
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"Rick Hughes" wrote
| Now I prefer to have some protection to prvent fire hazard, and
| thus have all light ccts through a 100mA RCD which are much less
| prone to nuiscance tripping.

But having all your lighting ccts through a single RCD - of whatever
rating - means you lose all lighting on the opening of one protective
device.

| My question is really pretty simple - do I protect Imeesrion
| and cooker or have them unprotected.

Well, if you need fire protection on your lighting circuits (and the only
reason I can think of in a normal dwelling would be if you have thatch roof
or straw bale walls), why would you *not* need it on cooker and immersion
circuits?

Lighting circuits are low current and use generously over-specified cables
in most cases, compared to cooker and immersion circuits which may be
operating at close to maximum capacity for extended periods of time. If
anything, they're more likely to cause a fire hazard through poor
connections overheating that a lighting circuit.

Owain







  #11   Report Post  
IMM
 
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"Owain" wrote in message
...
"Rick Hughes" wrote
| Now I prefer to have some protection to prvent fire hazard, and
| thus have all light ccts through a 100mA RCD which are much less
| prone to nuiscance tripping.

But having all your lighting ccts through a single RCD - of whatever
rating - means you lose all lighting on the opening of one protective
device.

| My question is really pretty simple - do I protect Imeesrion
| and cooker or have them unprotected.

Well, if you need fire protection on your lighting circuits (and the only
reason I can think of in a normal dwelling would be if you have thatch

roof
or straw bale walls), why would you *not* need it on cooker and immersion
circuits?

Lighting circuits are low current and use generously over-specified cables
in most cases, compared to cooker and immersion circuits which may be
operating at close to maximum capacity for extended periods of time. If
anything, they're more likely to cause a fire hazard through poor
connections overheating that a lighting circuit.


With modern downlighters and other types of lighting, the current drawn can
be quite substantial.


  #12   Report Post  
Alex
 
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"Handy Andy" bodgityourself.net wrote in message
...

"Rick Hughes" wrote in message
...
"Lurch" wrote in message

You seem to be doing well making up your own ways of wiring your house
so if I were you I'd just toss a coin!


I don't understand your comment ? is it just humor that I have missed,

or
do you really think I am making up my own rules ? I'm not.


Don't worry, he's just a silly little boy. He's probably just got himself

a
little City & Guilds and now he thinks he's an expert!


Says he who thinks his CU might have caught on fire because 'wires were a
bit tight'



Alex


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G&M
 
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"Rick Hughes" wrote in message
...

"tony sayer" wrote in message
I'd much prefer everything to be on a 30 ma RCD after all a 100 ma ones
not going to be much fun to try to trip through your body



Agree on life protection, and the 30mA RCD wil provide protection to the

AC
outlets, Radial and Ring Final circuits.

The Best Practise oft talked about is not to have lighting on a 30mA trip
duwe to nusisance tripping, many on this group for example have advocated

no
protection for light circuits.


I stuggle with this. When a light blows here it takes out the circuit trip
but has never to my knowledge tripped our 30mA RCD. Is there something
system dependent here ?


  #14   Report Post  
tony sayer
 
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In article , G&M
writes

"Rick Hughes" wrote in message
...

"tony sayer" wrote in message
I'd much prefer everything to be on a 30 ma RCD after all a 100 ma ones
not going to be much fun to try to trip through your body



Agree on life protection, and the 30mA RCD wil provide protection to the

AC
outlets, Radial and Ring Final circuits.

The Best Practise oft talked about is not to have lighting on a 30mA trip
duwe to nusisance tripping, many on this group for example have advocated

no
protection for light circuits.


I stuggle with this. When a light blows here it takes out the circuit trip
but has never to my knowledge tripped our 30mA RCD. Is there something
system dependent here ?



There are some misconceptions about what an RCD actually does, well
modern ones.

An RCD (Residual current device) is designed to trip when there is a
current UNBALANCE between what is coming IN and what is going OUT on the
circuit it is protecting.

Now under normal operating conditions it matters not how much current is
flowing through the device, provided that its within its ratings like 64
amps for example.

Now as long as the current in, through say the live terminal, is matched
by the current out through the neutral terminal, then all is fine. Now
if for some reason there is more going in than coming out above the
nominal tripping current, say 30 ma ,actually slightly less, then the
device will trip.

Supposing that you inadvertently touch the live wire somewhere in the
house, assuming that that particular circuit is protected by the RCD,
then there will be an element of current flowing through your body to
earth. Now if that current that you are "conducting" is greater than the
current trip level, then off goes the trip because it has detected less
current flowing OUT than the current coming IN hence an UNBALANCE!.

It doesn't even matter if you touch earth or not or even if your house
is on an earth or not, or even if there is no earth on your system, it
doesn't matter as far as the trip is concerned. The reason is that the
neutral line is earthed at the substation transformer, be that in a
substation proper, or on a pole mounted transformer. Due to this if say
you touch a live wire then you shunt some of the current via the trip
and the earth mass back to the substation and if that value of current
is greater than the trip level off will go the trip!.

Its down to a current unbalance through the trip, simple as that. If for
instance a cooker element develops some leakage between the element
casing then some current will flow to earth and if that current is
greater than the tripping unbalance current then off the trip will go.
Sometimes you may experience this as the element will only do this when
it gets hot and then the insulation will breakdown and the trip current
will be exceeded!.

Another "*******" problem comes if there is a short betwixt the neutral
line and the earth line in the wiring somewhere. Normally if most every
thing is switched off then no tripping, but as soon as some appliances
are switched on then off the trip will go!. The reason is that under
light or no load conditions the trip will be sensing an unbalance state
so the current flowing in will match the current out, because some of
the current will flow back along the neutral lead through the trip and
some direct to earth through the earth wire. Now as soon as some load is
put on the system and the current flowing through the combined earth and
neutral return starts to increase then the trip will sense that there is
more coming in than going out via the "authorised" route via the RCD, so
a unbalance exists and as soon as this exceeds the trip current off
again.

In my opinion all the wiring in your gaff should be on a 30ma trip
because there might be the time when you are working on something and
forget to turn it off, or pull the wrong fuse or something similar I
wouldn't fancy trying to trip a 100 ma trip with my body either thanks!.

So as long as you see the trip as a device that monitors the current in
and the current out are in balance, then all is fine if however there is
a difference between these two values greater than the trip level, then
the device will operate.

Hope this helps....
--
Tony Sayer

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Lurch
 
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On Mon, 9 Aug 2004 14:42:28 +0000 (UTC), "Rick Hughes"
strung together this:

I don't understand your comment ? is it just humor that I have missed, or
do you really think I am making up my own rules ?


Bit of both.

If I were me, which I am, I'd put the immersion and cooker on no
protection, which I did.
OOI, why do you think you need 3 classes of protection?


No protection - Alarm, Home Automation, Fridge & Freezer
100mA - fire protection
30mA - life protection

Fair enough, just wondered. Nothing wrong with it, just don't see
domestic installations, or many installations in general come to think
of it, with 3 classes of protection.
--

SJW
A.C.S. Ltd


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Handy Andy
 
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"Alex" wrote in message
...
"Handy Andy" bodgityourself.net wrote in message
...

"Rick Hughes" wrote in message
...
"Lurch" wrote in message

You seem to be doing well making up your own ways of wiring your

house
so if I were you I'd just toss a coin!

I don't understand your comment ? is it just humor that I have

missed,
or
do you really think I am making up my own rules ? I'm not.


Don't worry, he's just a silly little boy. He's probably just got

himself
a
little City & Guilds and now he thinks he's an expert!


Says he who thinks his CU might have caught on fire because 'wires were a
bit tight'

Do try to read things properly.


  #17   Report Post  
Lurch
 
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On Mon, 9 Aug 2004 14:51:52 +0000 (UTC), "Rick Hughes"
strung together this:

The Best Practise oft talked about is not to have lighting on a 30mA trip
duwe to nusisance tripping, many on this group for example have advocated no
protection for light circuits.

Now I prefer to have some protection to prvent fire hazard, and thus have
all light ccts through a 100mA RCD which are much less prone to nuiscance
tripping.

You've obviously mis-read something somewhere. RCD=earth fault
protection, MCB=overload protection.

RCD's work by measuring the amount of current on the live cables and
ensuring they are equal, if they aren't, by an amount determined by
it's rating, then it assumes the current has gone to earth, either
through a faulty appliance or you, and trips.

A light bulb isn't earthed so wouldn't take out an RCD. You're
thinking of MCB's which protect final circuits, light bulbs can easily
take out type B MCB's as the bulb takes a surge of current as it blows
which overloads the circuit but by fitting a type C or a HRC fuse to
the lighting circuits this nuisance tripping can be reduced.

Type C MCB's and HRC fuses react slower so won't always trip\blow.

If that's what you based the 3 classes of protection on then you might
not be the best person to be doing this project.

My question is really pretty simple - do I protect Imeesrion and cooker or
have them unprotected.
AFAIK the regs do not require protection, but that doen't mean i t's best
practise.

The immersion doesn't require RCD protection. The cooker doesn't
either, unless the manufacturers instructions say so or if the switch
incorporates a socket that can be reasonably expected to supply
portable equipment outdoors.
--

SJW
A.C.S. Ltd
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Rick Hughes
 
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"Owain" wrote in message
...
"Rick Hughes" wrote
| Now I prefer to have some protection to prvent fire hazard, and
| thus have all light ccts through a 100mA RCD which are much less
| prone to nuiscance tripping.

But having all your lighting ccts through a single RCD - of whatever
rating - means you lose all lighting on the opening of one protective
device.



Allo ny Hall & stairwell (and smoke detectors) are on non protected
circuit - so will have some light.

| My question is really pretty simple - do I protect Imeesrion
| and cooker or have them unprotected.

Well, if you need fire protection on your lighting circuits (and the only
reason I can think of in a normal dwelling would be if you have thatch

roof
or straw bale walls), why would you *not* need it on cooker and immersion
circuits?

Lighting circuits are low current and use generously over-specified cables
in most cases, compared to cooker and immersion circuits which may be
operating at close to maximum capacity for extended periods of time. If
anything, they're more likely to cause a fire hazard through poor
connections overheating that a lighting circuit.


appreciate your comments.


  #19   Report Post  
Rick Hughes
 
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"IMM" wrote in message ...
"Owain" wrote in message
...


With modern downlighters and other types of lighting, the current drawn

can
be quite substantial.


fair point except for a sprinkling of fl tubes juat about everything else is
a LovVo downlight or uplight.



  #20   Report Post  
Lurch
 
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On Mon, 9 Aug 2004 20:42:25 +0100, tony sayer
strung together this:

Now as long as the current in, through say the live terminal, is matched
by the current out through the neutral terminal,


Wrong way round, out from the RCD through the live and back in through
the neutral.

I wouldn't fancy trying to trip a 100 ma trip with my body either thanks!.

I have on occasions tried overloading circuits fed by 30A fuse wire,
using just myself as 'the load' and failed, and I've never managed to
trip an RCD by using me as a load across live and earth.
--

SJW
A.C.S. Ltd


  #21   Report Post  
Rick Hughes
 
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"tony sayer" wrote in message
...
In article , G&M
writes

"Rick Hughes" wrote in message
...

"tony sayer" wrote in message
I'd much prefer everything to be on a 30 ma RCD after all a 100 ma

ones
not going to be much fun to try to trip through your body


Agree on life protection, and the 30mA RCD wil provide protection to

the
AC
outlets, Radial and Ring Final circuits.

The Best Practise oft talked about is not to have lighting on a 30mA

trip
duwe to nusisance tripping, many on this group for example have

advocated
no
protection for light circuits.


I stuggle with this. When a light blows here it takes out the circuit

trip
but has never to my knowledge tripped our 30mA RCD. Is there something
system dependent here ?



There are some misconceptions about what an RCD actually does, well
modern ones.

An RCD (Residual current device) is designed to trip when there is a
current UNBALANCE between what is coming IN and what is going OUT on the
circuit it is protecting.

Now under normal operating conditions it matters not how much current is
flowing through the device, provided that its within its ratings like 64
amps for example.

Agreeed 100%, but it's the leaqkage that causes the nusianace trips I would
assume.


instance a cooker element develops some leakage between the element
casing then some current will flow to earth and if that current is
greater than the tripping unbalance current then off the trip will go.

So my question on whether to protect a cooker is a valid one?


Sometimes you may experience this as the element will only do this when
it gets hot and then the insulation will breakdown and the trip current
will be exceeded!.

Another "*******" problem comes if there is a short betwixt the neutral
line and the earth line in the wiring somewhere.




I had such a fault on a previous house .... trip would operate once a week
or so .... after hours and hours of fun, eventually could re-create it as
soon as I put a high current on the ring (toaster & kettle) eventually found
that SWMBO after loosening a socket for painting some months previous - had
crushed a neutral wire under the socket plate screw ...

If I didn't have an RCD on the cct ..... when would I have found the fault
? when it burst into flames ?

Rick


  #22   Report Post  
Rick Hughes
 
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"Lurch" wrote in message

You've obviously mis-read something somewhere. RCD=earth fault
protection, MCB=overload protection.



No not at all .... I refer to nusicance trippingt of RCD that is nothing at
all to do with overload protection.



A light bulb isn't earthed so wouldn't take out an RCD.



Wrong here I'm sorry I referred to lighting circuits not to light bulbs,
(but it could just be the sequence you are reading mail in) ... floreescents
are earthed and it is these that typically give the nuiscence tripping
reports, again an RCD would actiuvate not an MCB as there is an imbalance
but no overload.

You're thinking of MCB's which protect final circuits,


No I'm not - I am specifically referring to RCD protcetion.


If that's what you based the 3 classes of protection on then you might
not be the best person to be doing this project.


The 3 classes of protection are deliberate .... unprotrected, fire
protection and life protection - all defined separte calssses of
protection - exactly what I want on this project.

Do you have a comment on my qusetion ? I would be interested in your answer.

Q was - To RCD protect a cooker and immersion or not .... the regs do not
require it, but that may not be best practise.


Rick


  #23   Report Post  
Rick Hughes
 
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"G&M" wrote in message news:cf8hs0$8b2

I stuggle with this. When a light blows here it takes out the circuit

trip
but has never to my knowledge tripped our 30mA RCD. Is there something
system dependent here ?



My 'reading' and I'll admit it's not personal experince is that some
lights, such as Floureseents can have leakage which can cause a 30mA trip to
activate ... whether this is on switch on , or when choke gets hot, I'm not
sure.



Rick


  #24   Report Post  
Lurch
 
Posts: n/a
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On Mon, 9 Aug 2004 21:28:10 +0000 (UTC), "Rick Hughes"
strung together this:

Wrong here I'm sorry I referred to lighting circuits not to light bulbs,
(but it could just be the sequence you are reading mail in) ... floreescents
are earthed and it is these that typically give the nuiscence tripping
reports, again an RCD would actiuvate not an MCB as there is an imbalance
but no overload.

Ah, with you now. Not heard that one too often, the lightbulb scenario
I presentede is a far more common aoccurence so I assumed that it was
that you were referring to.
TBH, all circuits are likely to trip at some time or other. I don't
think the technical basis of your plans is particularly sound. I think
with a bit more electrical knowledge and experience you would see this
whole ting as a 'fiasco', (as I do)!

The 3 classes of protection are deliberate .... unprotrected, fire
protection and life protection - all defined separte calssses of
protection - exactly what I want on this project.

Do you have a comment on my qusetion ? I would be interested in your answer.

Q was - To RCD protect a cooker and immersion or not .... the regs do not
require it, but that may not be best practise.

I've answered that, twice. Only you can decide how much additional
protection over and above the regs you require.
I think from what you've said you'd be better off with RCBO's for all
final circuits, then all circuits have their own RCD and MCB so any
earth fault or overload will affect that circuit only.
--

SJW
A.C.S. Ltd
  #25   Report Post  
Lurch
 
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On Mon, 9 Aug 2004 21:28:10 +0000 (UTC), "Rick Hughes"
strung together this:

Agreeed 100%, but it's the leaqkage that causes the nusianace trips I would
assume.

Only if the leakage is not about to cause a fire\damage to equiopment
etc... If it goes off for a reason, i.e. a fault, then it's not a
nuisance trip. If it goes off for no apparent reason then it is a
nuisance trip.

instance a cooker element develops some leakage between the element
casing then some current will flow to earth and if that current is
greater than the tripping unbalance current then off the trip will go.

So my question on whether to protect a cooker is a valid one?

Yes, as is any question that involves the words "shall I RCD protect
my"?
In your case, it may or may not be neccesary to install an RCD on the
cooker circuit, if it does require one then that's your answer, if it
doesn't require one then you can fit one if you want some additional
protection.

If I didn't have an RCD on the cct ..... when would I have found the fault
? when it burst into flames ?

No, it would probably have never done anything at all. If you are on
PME, (which I seem to think you are), then neutral and earth are
connected together in the service cutout, and that hasn't burst into
flames has it?
Having the neutral and earth connected together throughout the house
on a PME system would never cause any problems whatsoever.
--

SJW
A.C.S. Ltd


  #26   Report Post  
G&M
 
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Default RCD or not to RCD ...


"Rick Hughes" wrote in message
...

"G&M" wrote in message news:cf8hs0$8b2

I stuggle with this. When a light blows here it takes out the circuit

trip
but has never to my knowledge tripped our 30mA RCD. Is there something
system dependent here ?


My 'reading' and I'll admit it's not personal experince is that some
lights, such as Floureseents can have leakage which can cause a 30mA trip

to
activate ... whether this is on switch on , or when choke gets hot, I'm

not
sure.


Possibly. But I've done same as Tony and put everything on a pair of 30mA
RCD CUs.


  #27   Report Post  
G&M
 
Posts: n/a
Default RCD or not to RCD ...


"Rick Hughes" wrote in message
...
Q was - To RCD protect a cooker and immersion or not .... the regs do

not
require it, but that may not be best practise.


I can see no reason not to RCD protect these. Even if there was a nuisance
trip, the worse that can happen is a cold supper or bath.


  #28   Report Post  
Owain
 
Posts: n/a
Default RCD or not to RCD ...

"G&M" wrote
| The Best Practise oft talked about is not to have lighting on a
| 30mA trip duwe to nusisance tripping, many on this group for
| example have advocated no protection for light circuits.
| I stuggle with this. When a light blows here it takes out the
| circuit trip but has never to my knowledge tripped our 30mA RCD.
| Is there something system dependent here ?

The point is not the lighting circuit RCD tripping on a lighting circuit
fault, which is sensible, but a *whole-house* RCD tripping, taking out all
the lights and leaving you in darkness, because of an unrelated fault.

As "dmc" pointed out, and I followed up on 10/7/2003:

| If a whole-house RCD on consumer unit trips on the day that you leave
| for 3 weeks holiday - just as you shut the door. You return to a very
| defrosted rather unpleasant freezer...

The smell should help deter burglars, which is as well as the burglar alarm
battery probably died on the third day.

| Opposite - it trips mid winter while you are away for a bit. Lack of
| heating causes burst pipes and floods the place...

And the video doesn't record all the episodes of Who Wants To Be A Convicted
Fraudster you were looking forward to watching on your return.

Even if the RCD just trips during the night, it takes out the baby-alarm so
you don't hear the wean choking to death during the night, and the
clock-radio dies so you sleep in late. In your rush to phone an ambulance
and your boss, you find the cordless phone won't worth without power to the
base unit, so you rush downstairs, forgetting that the light won't come on
when you press the switch as you simultaneously step off the landing into
space. The noise of you landing at the bottom of the stairs wakes the missus
who starts giving you grief, and after you've dragged your broken leg into
the kitchen you find the electric kettle won't work for the missus' early
morning tea and croissants. After trying a saucepan on the gas, and
giving up on the electric spark ignition, you drag your broken leg through
to the lounge, turn on the gas fire which has crankomatic piezo ignition,
light a newspaper spill, carry that through to the kitchen, and wave it
vaguely at the gas under a saucepan.

Then you drag yourself upstairs again to the missus to explain why there's
no hot water in the en-suite. It's only after taking a
massive-curry-the-night-before dump you realise the saniflo won't flush
either.

Meanwhile, the unlit gas has eventually reached the newspaper spill you
hurriedly threw down in the kitchen, and is no longer unlit. With a gentle
wooomf you now have the next location for BBC Fire Detectives (Tuesdays
10.35pm BBC1) in your very own home.

Upstairs you remain unaware of this, as the weekend you were determined to
fit the smoke detectors Bodgit & Queue were out of stock of the battery
back-up ones so you economised and got the mains-only, and you're busy
trying to pacify the missus and revive the bairn with some warmth. The only
heating you can think of is that old gas bottle heater in the spare room.
Getting the nozzle on in the dark is more difficult than you expected, but
you know it's still got gas in it because you can hear it hissing.

At this point the missus decides that if she can't have an early morning
cuppa, hot bath, or even the telly on to ogle Kilroy, she might as well have
some nicotine and reaches out for the cigarettes and a two-furra-pound gas
lighter.

".... to be continued"

Owain



  #29   Report Post  
Andy Hall
 
Posts: n/a
Default RCD or not to RCD ...

On Mon, 9 Aug 2004 23:14:31 +0100, "G&M"
wrote:


"Rick Hughes" wrote in message
...
Q was - To RCD protect a cooker and immersion or not .... the regs do

not
require it, but that may not be best practise.


I can see no reason not to RCD protect these. Even if there was a nuisance
trip, the worse that can happen is a cold supper or bath.




There are a few exceptions though.

It's a good idea to have at least some lighting on unprotected
circuits. Then if the RCD trips, you are not also plunged into
darkness.

For my workshop, I have a separate supply and CUs and have a 30A
circuit for a woodworking machine. This has an electronic 3phase
convertor inside and the manufacturer of this specifies a 100mA RCD,
AIUI because of the filter arrangement. It also requires a type C
MCB, as does the dust extractor because of high initial load currents.





..andy

To email, substitute .nospam with .gl
  #30   Report Post  
Lurch
 
Posts: n/a
Default RCD or not to RCD ...

On Mon, 9 Aug 2004 23:14:31 +0100, "G&M"
strung together this:

I can see no reason not to RCD protect these. Even if there was a nuisance
trip, the worse that can happen is a cold supper or bath.

Not really, if the RCD tripped then you would get a cold supper _and_
cold bath if you put the immersion and cooker on the RCD. I would say
the immersion and cooker would cause more nuisance tripping which is
exactly what the deluded chap who started this is trying to avoid
which only further convinces me that he really shouldn't be doing
this.
I think RCBO's would be the only way to prevent nuisance tripping
affecting anything else, he's coming in at this from the wrong angle.
--

SJW
A.C.S. Ltd


  #31   Report Post  
RichardS
 
Posts: n/a
Default RCD or not to RCD ...

"Lurch" wrote in message
...
On Mon, 9 Aug 2004 14:51:52 +0000 (UTC), "Rick Hughes"
strung together this:

The Best Practise oft talked about is not to have lighting on a 30mA trip
duwe to nusisance tripping, many on this group for example have advocated

no
protection for light circuits.

Now I prefer to have some protection to prvent fire hazard, and thus have
all light ccts through a 100mA RCD which are much less prone to nuiscance
tripping.

You've obviously mis-read something somewhere. RCD=earth fault
protection, MCB=overload protection.

RCD's work by measuring the amount of current on the live cables and
ensuring they are equal, if they aren't, by an amount determined by
it's rating, then it assumes the current has gone to earth, either
through a faulty appliance or you, and trips.

A light bulb isn't earthed so wouldn't take out an RCD. You're
thinking of MCB's which protect final circuits, light bulbs can easily
take out type B MCB's as the bulb takes a surge of current as it blows
which overloads the circuit but by fitting a type C or a HRC fuse to
the lighting circuits this nuisance tripping can be reduced.

Type C MCB's and HRC fuses react slower so won't always trip\blow.

If that's what you based the 3 classes of protection on then you might
not be the best person to be doing this project.

My question is really pretty simple - do I protect Imeesrion and cooker

or
have them unprotected.
AFAIK the regs do not require protection, but that doen't mean i t's

best
practise.

The immersion doesn't require RCD protection. The cooker doesn't
either, unless the manufacturers instructions say so or if the switch
incorporates a socket that can be reasonably expected to supply
portable equipment outdoors.



Been reading through this thread with some interest.

Correct me if I'm wrong, but I can't see the 100mA RCD offering any useful
protection against fire hazard at all in this installation, and nothing
useful against electrocution.

Rick correctly identifies the need for 30mA RCD as protection against shock
hazard for socket circuits, the rationale of course being that if you touch
a live part then as soon as the current through you to earth reaches 30mA
the protection trips, disconnecting the supply. Fine, so we certainly
install 30mA non-time-delay on all circuits likely to be used for any
external portable appliances, and personal preference dictates whether other
socket circuits (such as upstairs) are similarly protected. Some appliances
within various zones of the bathroom will also require this protection.

This doesn't provide any protection against circuit overload (one of the
causes of fire in fixed wiring), though, because unless it's implemented as
a RCBO then this isn't it's job - the MCB takes care of that. As I see it,
the other likely cause of fires (loose connections in the circuit giving
localised high resistance and therefore heat generation) isn't protected
against by either the MCB or RCD.

So, what's the 100mA RCD for? It can't be against electrocution, because
100mA represents a fatal shock hazard. Whole-installation 100mA time-delay
is used in TT systems due to problems with the earth impedance and
disconnection times for MCBs under fault-load conditions.

I don't see that earth leakage is a significant fire hazard. The most
likely ones are chronic circuit overload (MCB's job to protect against this)
or loose connections (and neither will protect against this, unless the
insulation breaks down to such an extent that there is significant leakage
to earth, and once this happens then it's likely to produce a fault current
that the MCB will protect against).

Not sure about reports of "nuisance tripping" that the OP talks about
either. All the ones I have read have been concerned with blowing bulbs
taking out the MCB due to high fault currents generated. Either that, or
deprecated whole-house 30mA RCDs taking out all the lighting circuits due to
earth leakage from either a portable appliance on a socket circuit, or
mineral insulated heating elements leaking to earth. There's a jolly good
reason why whole-house 30mA protection is deprecated.

Finally, for the OP, there could be a very good reason coming up on 1/1/05
why you shouldn't start introducing unneccessary RCD protection. (as
opposed to "necessary" RCD protection - I'm NOT advocating that they are
always unneccessary !)

My reading of Part P is that RCDs are always "notifiable" devices for the
DIYer. So, if your superfluous 100mA RCD develops a fault, and you're the
type to obey the letter of the law, you've just landed yourself with a bill
for a self-certing spark to change it, or for BCO approval.




--
Richard Sampson

email me at
richard at olifant d-ot co do-t uk


  #32   Report Post  
G&M
 
Posts: n/a
Default RCD or not to RCD ...


"Andy Hall" wrote in message
...
On Mon, 9 Aug 2004 23:14:31 +0100, "G&M"
wrote:


"Rick Hughes" wrote in message
...
Q was - To RCD protect a cooker and immersion or not .... the regs do

not
require it, but that may not be best practise.


I can see no reason not to RCD protect these. Even if there was a

nuisance
trip, the worse that can happen is a cold supper or bath.




There are a few exceptions though.

It's a good idea to have at least some lighting on unprotected
circuits. Then if the RCD trips, you are not also plunged into
darkness.


I would argue that my solution of two smaller CUs each with RCDs plus some
careful wiring around the house to ensure there is always a working light
nearby is safer. If room lights are on one but staircase and exits on the
other you'll never be far from a working light switch.

In any case one should either have some form of emergency lighting or a
torch nearby for power cuts which are frequent where we are and are
predicted to get worse everywhere.



  #33   Report Post  
tony sayer
 
Posts: n/a
Default RCD or not to RCD ...

In article , Lurch
writes
On Mon, 9 Aug 2004 20:42:25 +0100, tony sayer
strung together this:

Now as long as the current in, through say the live terminal, is matched
by the current out through the neutral terminal,


Wrong way round, out from the RCD through the live and back in through
the neutral.


Well that is another way of saying the same thing. Mind you a drawing
would be worth a 10000 words in this instance

I wouldn't fancy trying to trip a 100 ma trip with my body either thanks!.

I have on occasions tried overloading circuits fed by 30A fuse wire,
using just myself as 'the load' and failed, and I've never managed to
trip an RCD by using me as a load across live and earth.


Something wrong there...your still here to tell the tale!....

--
Tony Sayer

  #34   Report Post  
tony sayer
 
Posts: n/a
Default RCD or not to RCD ...

In article , Rick Hughes
writes

"tony sayer" wrote in message
...
In article , G&M
writes

"Rick Hughes" wrote in message
...

"tony sayer" wrote in message
I'd much prefer everything to be on a 30 ma RCD after all a 100 ma

ones
not going to be much fun to try to trip through your body


Agree on life protection, and the 30mA RCD wil provide protection to

the
AC
outlets, Radial and Ring Final circuits.

The Best Practise oft talked about is not to have lighting on a 30mA

trip
duwe to nusisance tripping, many on this group for example have

advocated
no
protection for light circuits.

I stuggle with this. When a light blows here it takes out the circuit

trip
but has never to my knowledge tripped our 30mA RCD. Is there something
system dependent here ?



There are some misconceptions about what an RCD actually does, well
modern ones.

An RCD (Residual current device) is designed to trip when there is a
current UNBALANCE between what is coming IN and what is going OUT on the
circuit it is protecting.

Now under normal operating conditions it matters not how much current is
flowing through the device, provided that its within its ratings like 64
amps for example.

Agreeed 100%, but it's the leaqkage that causes the nusianace trips I would
assume.


Yes its the unbalance between what's going in and coming out any
difference in excess of the rated trip current then disconnect!..


instance a cooker element develops some leakage between the element
casing then some current will flow to earth and if that current is
greater than the tripping unbalance current then off the trip will go.

So my question on whether to protect a cooker is a valid one?


Sometimes you may experience this as the element will only do this when
it gets hot and then the insulation will breakdown and the trip current
will be exceeded!.

Another "*******" problem comes if there is a short betwixt the neutral
line and the earth line in the wiring somewhere.




I had such a fault on a previous house .... trip would operate once a week
or so .... after hours and hours of fun, eventually could re-create it as
soon as I put a high current on the ring (toaster & kettle) eventually found
that SWMBO after loosening a socket for painting some months previous - had
crushed a neutral wire under the socket plate screw ...

If I didn't have an RCD on the cct ..... when would I have found the fault
? when it burst into flames ?


Doubt it would have done that.....

Rick



--
Tony Sayer

  #35   Report Post  
tony sayer
 
Posts: n/a
Default RCD or not to RCD ...

In article , G&M
writes

"Rick Hughes" wrote in message
...
Q was - To RCD protect a cooker and immersion or not .... the regs do

not
require it, but that may not be best practise.


I can see no reason not to RCD protect these. Even if there was a nuisance
trip, the worse that can happen is a cold supper or bath.



Well its very debatable. It can show up faults in leakage between the
wires in cooker and immersion elements before they finally break down
which can be a good thing. I've never had a problem with it causing a
cold supper or bath tho. Course there might be a time when you
inadvertently connect yourself to the cooker or immersion circuits then
perhaps you'll wish you had;!..

The possible best bet is to use combined overcurrent and leakage units
to protect each circuit at the CU board.....
--
Tony Sayer



  #36   Report Post  
Rick Hughes
 
Posts: n/a
Default RCD or not to RCD ...


"Lurch" wrote in message
...

exactly what the deluded chap who started this is trying to avoid
which only further convinces me that he really shouldn't be doing
this.




Nice to know that the standard of help on this group has deteriorated, it
seems whoever asks a question now, gets insulted.
I am neither deluded, and MILO consider myself completely competent to
undertake this project - I have wired out many installations, all I asked
was a best practise question - at no time have I suggested anything
dangerous ort contrary to the regs - so why do you feel it necessary to
resort to insults ?

Thanks to those who did give valuable comments.

Rick


  #37   Report Post  
Lurch
 
Posts: n/a
Default RCD or not to RCD ...

On Tue, 10 Aug 2004 11:05:45 +0100, tony sayer
strung together this:

Well that is another way of saying the same thing. Mind you a drawing
would be worth a 10000 words in this instance


Quite.

I have on occasions tried overloading circuits fed by 30A fuse wire,
using just myself as 'the load' and failed, and I've never managed to
trip an RCD by using me as a load across live and earth.


Something wrong there...your still here to tell the tale!....


I'm bionic me. :-)
--

SJW
A.C.S. Ltd
  #38   Report Post  
Lurch
 
Posts: n/a
Default RCD or not to RCD ...

On Tue, 10 Aug 2004 11:10:44 +0100, tony sayer
strung together this:

The possible best bet is to use combined overcurrent and leakage units
to protect each circuit at the CU board.....


Exactly what I've suggested on more than one occasion during the
course of this thread.
--

SJW
A.C.S. Ltd
  #39   Report Post  
Lurch
 
Posts: n/a
Default RCD or not to RCD ...

On Tue, 10 Aug 2004 11:35:33 +0000 (UTC), "Rick Hughes"
strung together this:

Thanks to those who did give valuable comments.

That would be some of my comments then, which you chose to ignore. If
you come on here, ask a question, then tell everyone your way is
better then there's not really much point asking the question.
As I have said several times, YOUR BEST BET WOULD BE TO USE RCBO'S ON
ALL THE CIRCUITS.
--

SJW
A.C.S. Ltd
  #40   Report Post  
tony sayer
 
Posts: n/a
Default RCD or not to RCD ...

In article , Lurch
writes
On Tue, 10 Aug 2004 11:10:44 +0100, tony sayer
strung together this:

The possible best bet is to use combined overcurrent and leakage units
to protect each circuit at the CU board.....


Exactly what I've suggested on more than one occasion during the
course of this thread.


Back to the cooker circuit protection business, it appears that some
cooker outlet plates have a 13 amp outlet on them which isn't or won't
normally be protected into which you could plug a duff appliance etc
with attendant shock risk?.....
--
Tony Sayer


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