Just checking it's not just me, but I think my new induction hob is faulty!!

It is connected to the RCD consumer unit (Because the cable that feeds it
was feeding the oven, and was already connected there before you ask!)

When turning on the local isolator near the hob, the RCD trips.
If I reset the RCD it will trip either immediately or after a few seconds
If I isolate the hob (Via the double pole isolator, OR by the single pole
MCB in the CU) the tripping stops.

I tried disconnecting it's earth (and keeping away from it!), and the
tripping stopped
If I measure the current from it's earth wire to earth, the RCD trips (no
surprise there then!)

If I measure the current between its earth wire and Neutral, I am getting
about 2.9A
If I measure the voltage, it is about 120v

Now when it was first turned on (this afternoon), it tripped the RCD
immediately, then after a few resets, it seemed to settle down - I even
boiled some water on it to test it.
I thought it must have been a bit damp or something (Although it has been
sitting in a warm house for about two weeks!)

We went out and set the alarm as we would usually do, only to be called by
the alarm about 10 minutes after leaving the house telling me my power had
gone off (The hob had been powered up, but "off" for a good hour)
Now the RCD refuses to stay on with the hob powered up - This is when I
tested it.

Do I send it back, or connect it to the non RCD CU!
(I personally would think 120v @ 2.9A leakage to earth is not too clever, so
would think it is faulty, but would just like to see what others think)

....If I hadn't have connected it to an RCD, then this fault (assuming it is)
would never have been noticed (Unless it got worse, and tried to pull more
than 32A, tripping the MCB!)
If the earth had then become disconnected somewhere between it and the CU,
then someone touching it may have got a packet from it.

Is not RCD protecting things like this really as wise as people make out?
(The same goes for an old dishwasher we used to own, the RCD kept tripping
with this when the door was opened - turned out to be a neutral wire worn
through, touching the chassis - again this would not have been highlighted
without having an RCD...

Sparks (who wants to RCD everything possible, now more than ever!)

Sparks wrote:

Just checking it's not just me, but I think my new induction hob is faulty!!
[...]
If I measure the current between its earth wire and Neutral, I am getting
about 2.9A

That's a bit ambiguous - do you mean the current that flows through your
meter with the meter leads connected between the neutral and earth
terminals, with the earth connected? (Or if not, then what exactly?) A
neutral-earth s/c current measurement like that isn't very helpful -
what you need to do is to measure the current in the earth wire of the
appliance. IOW connect the earth wire from the hob to the earth
terminal of the cooker connection unit via your meter. Make sure the
appliance metalwork is floating, i.e. isn't connected to earth through
any other path. Now what earth leakage reading do you get? Anything
more than a couple of milliamps is a cause for concern.

If I measure the voltage, it is about 120v

Strongly suggestive of a mains filter (RFI filter) and perfectly normal.

****

What (if anything) do the instructions for the hob say about earth
leakage current and compatibility with RCDs? There is certainly no need
for an appliance like this to be RCD protected, and there's no advantage
in it being so (TT-earthed installations excepted). But that said, the
leakage from a single healthy domestic appliance should not be anywhere
near high enough to trip a 30 mA RCD.

Sparks (who wants to RCD everything possible, now more than ever!)

Why?

--
Andy

"Andy Wade" wrote in message
...
Sparks wrote:

Just checking it's not just me, but I think my new induction hob is
faulty!!
[...]
If I measure the current between its earth wire and Neutral, I am getting
about 2.9A

That's a bit ambiguous - do you mean the current that flows through your
meter with the meter leads connected between the neutral and earth
terminals, with the earth connected? (Or if not, then what exactly?) A
neutral-earth s/c current measurement like that isn't very helpful - what
you need to do is to measure the current in the earth wire of the
appliance. IOW connect the earth wire from the hob to the earth terminal
of the cooker connection unit via your meter.

Yep, this is exactly what I did Disconnected the earth from the connection
plate, then clipped one test lead to it, and the other the Neutral in the
connection plate (as connecting it to the earth in the plate trips the RCD)
I also tried clipping the test lead to the metal body of the hob and the
neutral in the connection plate - both displayed the same results of 2.9A
flowing (The test lead produces small sparks when connected too)

Make sure the appliance metalwork is floating, i.e. isn't connected to
earth through any other path. Now what earth leakage reading do you get?
Anything more than a couple of milliamps is a cause for concern.

Hob is sitting on a granite worktop, and the parts touching this are plastic
anyway so is isolated pretty well!

If I measure the voltage, it is about 120v

Strongly suggestive of a mains filter (RFI filter) and perfectly normal.

****

What (if anything) do the instructions for the hob say about earth leakage
current and compatibility with RCDs?

Nothing!

There is certainly no need for an appliance like this to be RCD protected,
and there's no advantage in it being so (TT-earthed installations
excepted). But that said, the leakage from a single healthy domestic
appliance should not be anywhere near high enough to trip a 30 mA RCD.

Sparks (who wants to RCD everything possible, now more than ever!)

Why?

Because if the hob is faulty, as I expect it is, this fault would not have
been identified as the leakage would not trip the 32A MCB - Say at some
point the earth gets disconnected for whatever reason - the chassis of this
hob will be ready to supply a user with 2.9A @ 120v - and as it is only
leaking 2.9A no protection device would trip.
Not only this, but I would be wasting nearly 350w of power 24/7!
(That's 8.4KWh per day or 3068KWh per year (That's nearly £250 per year
leakage!!)

Surely it has to be safer to provide more protection than is actually
needed?
(If not, please tell me why!!)

Sparks...

"Andy Wade" wrote in message
...


If I measure the voltage, it is about 120v

Strongly suggestive of a mains filter (RFI filter) and perfectly normal.


It is possible that there is sufficient current flowing due to a mains
filter to trip the RCD. I've never been keen on suppressors between live
and earth because of the resultant reactive current whereas the neutral
current will be minimal.

Sparks wrote:

That's a bit ambiguous - do you mean the current that flows through your
meter with the meter leads connected between the neutral and earth
terminals, with the earth connected? (Or if not, then what exactly?) A
neutral-earth s/c current measurement like that isn't very helpful - what
you need to do is to measure the current in the earth wire of the
appliance. IOW connect the earth wire from the hob to the earth terminal
of the cooker connection unit via your meter.


Yep, this is exactly what I did Disconnected the earth from the connection
plate, then clipped one test lead to it, and the other the Neutral in the
connection plate (as connecting it to the earth in the plate trips the RCD)
I also tried clipping the test lead to the metal body of the hob and the
neutral in the connection plate - both displayed the same results of 2.9A
flowing (The test lead produces small sparks when connected too)

Whoa - not the way to go. Remember that when you have your meter set to
read current it looks like a short circuit across its probes (because it
is meant to be placed in series with a circuit - not across it like when
measuring voltage). If I understand what you have written correctly then
you have just created an earth neutral short. A sure recipe for tripping
a RCD in many cases. What sort of earthing do you have?

What you need to do is leave the live connected to the live, neutral to
the neutral, and then disconnect the earth. Now connect one test lead to
the earth connector on the appliance, and the other one to the earth
wire you just disconnected. That will let you measure the current
flowing to earth. You will probably need you meter on a lower range! You
may still get a trip when you do this - but you should be able to get a
reading for the leakage current. You could try turning off other RCD
protected circuits at the CU for the duration of the test to desensitise
the RCD (by removing other sources of imbalance/leakage) if required.

Sparks (who wants to RCD everything possible, now more than ever!)

Why?


Because if the hob is faulty, as I expect it is, this fault would not have
been identified as the leakage would not trip the 32A MCB - Say at some
point the earth gets disconnected for whatever reason - the chassis of this
hob will be ready to supply a user with 2.9A @ 120v - and as it is only
leaking 2.9A no protection device would trip.

If it is faulty, then passing 2.9A to earth would be a rather odd fault.
I expect you are simply measuring a large current flow between N and E
as a result of a slight difference between your earth and neutral
potentials. You would also not have been able to use the cooker to boil
your pan of water unless your RCD was also seriously knackered as well
since 2.9A should trip a 30mA threshold device rather rapidly.

(I take it it trips when you hit the test button?)

The 120V your read sounds like capacitive coupling. It is hard to think
of a fault in a typical suppression circuit on the input of an appliance
like this that would give you a 2.9A (i.e an 90 ish ohm resistance from
live to earth) that would not result in something in the filter going
bang in short order, or preventing the cooker from working.

Surely it has to be safer to provide more protection than is actually
needed?
(If not, please tell me why!!)

Several reasons - you have already found one of them. Heating devices
like this are often have high leakage currents (and we are talking
milliamps here - not amps), which means you are eating into the leakage
allowed by the RCD for all its protected circuits. The net result a
greater likelihood of nuisance trips. RCDs are designed to protect you
from electrocution in situations where there is a significant risk of
you making contact with live - i.e. portable appliances, with the worst
culprits being power tools used outside. For a permanently wired fixed
appliance this will be very unlikely and the risk is minimal.
Overcurrent protection and decent earthing gove you more than adequate
protection (unless you are on a TT installation).


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Yep, this is exactly what I did Disconnected the earth from the
connection plate, then clipped one test lead to it, and the other the
Neutral in the connection plate (as connecting it to the earth in the
plate trips the RCD)
I also tried clipping the test lead to the metal body of the hob and the
neutral in the connection plate - both displayed the same results of 2.9A
flowing (The test lead produces small sparks when connected too)

Whoa - not the way to go. Remember that when you have your meter set to
read current it looks like a short circuit across its probes (because it
is meant to be placed in series with a circuit - not across it like when
measuring voltage).

Yea, I know! I wanted to see the current flowing out of the earth

If I understand what you have written correctly then
you have just created an earth neutral short

Nope, if I plasced the test leads on the earthe on the supply earth and
supply ~Nuteral I would have done this

.. A sure recipe for tripping
a RCD in many cases. What sort of earthing do you have?

TN-S

What you need to do is leave the live connected to the live, neutral to
the neutral, and then disconnect the earth. Now connect one test lead to
the earth connector on the appliance, and the other one to the earth wire
you just disconnected.

That will trip the RCD, as the leakage is a lot more then 30mA

That will let you measure the current
flowing to earth.

As this method trips the RCD, I connected the probe the the Nuteral, as it
is at the same potential as earth

You will probably need you meter on a lower range! You
may still get a trip when you do this - but you should be able to get a
reading for the leakage current.

It trips as soon as the meter touches the copper, so I cant get a reading
quick enough!

You could try turning off other RCD
protected circuits at the CU for the duration of the test to desensitise
the RCD (by removing other sources of imbalance/leakage) if required.

Not really applicabalw with my test - it is definatly dumping 2.9A to earth!

Sparks (who wants to RCD everything possible, now more than ever!)

Why?


Because if the hob is faulty, as I expect it is, this fault would not
have been identified as the leakage would not trip the 32A MCB - Say at
some point the earth gets disconnected for whatever reason - the chassis
of this hob will be ready to supply a user with 2.9A @ 120v - and as it
is only leaking 2.9A no protection device would trip.

If it is faulty, then passing 2.9A to earth would be a rather odd fault. I
expect you are simply measuring a large current flow between N and E

Well, sort of - the disconnected earth of the hob and the N of the supply.

as a result of a slight difference between your earth and neutral
potentials. You would also not have been able to use the cooker to boil
your pan of water unless your RCD was also seriously knackered as well
since 2.9A should trip a 30mA threshold device rather rapidly.

I havent managed to measure the current when the RCD was staying on - so
mabe when this was happening it wasnt leaking quite so much!
The RCD is only about a year old - it was replaced as the pervious one went
well over sensertive

(I take it it trips when you hit the test button?)

Yep, and also when it was tested recently with an RCD tester )

In article ,
Fred wrote:

It is possible that there is sufficient current flowing due to a
mains filter to trip the RCD. I've never been keen on
suppressors between live and earth because of the resultant
reactive current whereas the neutral current will be minimal.

I've never understood why inline mains filters are
not two series chokes, a cap between L-N, and a
single cap from N to E.

--
Tony Williams.

On Thu, 21 Apr 2005 23:38:23 +0100, "Sparks" wrote:

Just checking it's not just me, but I think my new induction hob is faulty!!

Probably not.

When turning on the local isolator near the hob, the RCD trips.
If I reset the RCD it will trip either immediately or after a few seconds
If I isolate the hob (Via the double pole isolator, OR by the single pole
MCB in the CU) the tripping stops.

Induction hobs use high frequency and high current in the induction
coils. The switching electronics produce a lot of RF interference
which needs to be filtered to stop it getting back into the mains
supply.

A whole house RCD isn't fussed about where leakage is coming from and
if you have a number of noise filtered devices (such as computers,
printers, fax machines)on other circuits then each can leak about
3.5mA and it is likely that the RCD is already on the point of
tripping and the hob filter has just pushed it over the edge. Noise
filter leakage usually occurs whether the device is switched on
(using its own power switch) or not.

If I measure the current between its earth wire and Neutral, I am getting
about 2.9A

The current reading is irrelevant as has been pointed out.

If I measure the voltage, it is about 120v

Which is exactly as you would expect from a noise filtered device.

Do I send it back, or connect it to the non RCD CU!
(I personally would think 120v @ 2.9A leakage to earth is not too clever,

It isn't there. Your measurement ignored the neutral/earth voltage
difference. You would get similar results from any device in the
house.

would think it is faulty, but would just like to see what others think)

Connecting lots of things to a single 30mA RCD is a recipe for
nuisance tripping. You seem to have somewhat of an attachment to
RCD's so if you really want one connect the hob to a non-RCD circuit
and put a dedicated RCD on the hob alone.

Is not RCD protecting things like this really as wise as people make out?

Yes - whole house RCD's have killed more people than they have saved.
Increasing use of electronic devices and the increased need for RF
filtering means many houses now qualify as high protective conductor
current environments and should meet Sect 607 requirement.
(www.cda.org.uk/megab2/elecapps/pub142lo.pdf covers Sect 607 well).

Sparks (who wants to RCD everything possible, now more than ever!)

If you feel you must then use lots of individual RCD's at each device
you wish to protect.

--
Peter Parry.
http://www.wpp.ltd.uk/

"Peter Parry" wrote in message
...
On Thu, 21 Apr 2005 23:38:23 +0100, "Sparks" wrote:

If I measure the current between its earth wire and Neutral, I am getting
about 2.9A

The current reading is irrelevant as has been pointed out.

If I measure the voltage, it is about 120v

Which is exactly as you would expect from a noise filtered device.

Do I send it back, or connect it to the non RCD CU!
(I personally would think 120v @ 2.9A leakage to earth is not too clever,

It isn't there. Your measurement ignored the neutral/earth voltage
difference. You would get similar results from any device in the
house.

would think it is faulty, but would just like to see what others think)

Connecting lots of things to a single 30mA RCD is a recipe for
nuisance tripping. You seem to have somewhat of an attachment to
RCD's so if you really want one connect the hob to a non-RCD circuit
and put a dedicated RCD on the hob alone.


2.9A reactive current sounds far too much. In reality the earth will be
close to neutral potential so the measurement is a fair one in this case.
You wouldn't allow the earth to run at 120V would you? BTW I hope these are
AC measurements and not a DC ones!

I am certain that new standards would forbid such a poor power-factor or
such a reactive current to flow even from live to neutral.

I would ask the technical department at the manufacturer to reflect on the
measurements before doing anything further.

Sparks wrote:

a RCD in many cases. What sort of earthing do you have?


TN-S


What you need to do is leave the live connected to the live, neutral to
the neutral, and then disconnect the earth. Now connect one test lead to
the earth connector on the appliance, and the other one to the earth wire
you just disconnected.


That will trip the RCD, as the leakage is a lot more then 30mA

In which case can you move the supply to the non RCD side of the CU and
repeat the test?

As this method trips the RCD, I connected the probe the the Nuteral, as it
is at the same potential as earth

Your Neutral is very unlikely to be the *same* potential as earth -
close certainly, but given they are both going to be low impedance paths
there is a good chance a substantial current will be able to flow
between them if connected.

If it is faulty, then passing 2.9A to earth would be a rather odd fault. I
expect you are simply measuring a large current flow between N and E


Well, sort of - the disconnected earth of the hob and the N of the supply.

So that measurement is taken with the hob effectively not earthed at all
(i.e. the earth wire from the supply cable was not connected to anything)?

If you diconnect the hob and measure the resistance between L & E and E
& N and L & N (with all the switches/controls on the hob turned off),
what readings do you get?


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"Peter Parry" wrote in message
...
On Thu, 21 Apr 2005 23:38:23 +0100, "Sparks" wrote:

Just checking it's not just me, but I think my new induction hob is
faulty!!

Probably not.

Maybe you are miss understanding me, it is hard to explain everything
without trying pages and pages!...

When turning on the local isolator near the hob, the RCD trips.
If I reset the RCD it will trip either immediately or after a few seconds
If I isolate the hob (Via the double pole isolator, OR by the single pole
MCB in the CU) the tripping stops.

Induction hobs use high frequency and high current in the induction
coils. The switching electronics produce a lot of RF interference
which needs to be filtered to stop it getting back into the mains
supply.

But this is happening with the hob on standby.

A whole house RCD isn't fussed about where leakage is coming from and
if you have a number of noise filtered devices (such as computers,
printers, fax machines)on other circuits then each can leak about
3.5mA and it is likely that the RCD is already on the point of
tripping and the hob filter has just pushed it over the edge. Noise
filter leakage usually occurs whether the device is switched on
(using its own power switch) or not.

If I measure the current between its earth wire and Neutral, I am getting
about 2.9A

The current reading is irrelevant as has been pointed out.

Sorry, but you must be missing the point here!
I am NOT measuring between a connected Neutral and a connected earth, simply
a connected Neutral and the DISCONNECTED earth of the hob (Or the chassis of
the hob, with the Earth wire DISCONNECTED)

If I measure the voltage, it is about 120v

Which is exactly as you would expect from a noise filtered device.

Do I send it back, or connect it to the non RCD CU!
(I personally would think 120v @ 2.9A leakage to earth is not too clever,

It isn't there. Your measurement ignored the neutral/earth voltage
difference. You would get similar results from any device in the
house.

I would hope I wouldn't (as explained above)

would think it is faulty, but would just like to see what others think)

Connecting lots of things to a single 30mA RCD is a recipe for
nuisance tripping. You seem to have somewhat of an attachment to
RCD's so if you really want one connect the hob to a non-RCD circuit
and put a dedicated RCD on the hob alone.

So I will then get the 2.9A leaking to earth, 24/7 - Do I really want this?

Is not RCD protecting things like this really as wise as people make out?

Yes - whole house RCD's have killed more people than they have saved.

I never asked about whole house RCD's as I don't have one.
(I have a non RCD CU for things like the freezer, fridge, alarm, sub-main
feeds to further CU's in outbuildings (These of course have RCD protection
locally) and a separate RCD for the lighting circuit in the house, and
another one for socket's etc.


Sparks

"John Rumm" wrote in message
...
Sparks wrote:

a RCD in many cases. What sort of earthing do you have?


TN-S


What you need to do is leave the live connected to the live, neutral to
the neutral, and then disconnect the earth. Now connect one test lead to
the earth connector on the appliance, and the other one to the earth wire
you just disconnected.


That will trip the RCD, as the leakage is a lot more then 30mA

In which case can you move the supply to the non RCD side of the CU and
repeat the test?

I havent tried this, as I felt it was pointless due to the results of mu
tests
I can do this is it is really necassary, however it will be a bit of a mare
to do!

As this method trips the RCD, I connected the probe the the Nuteral, as
it is at the same potential as earth

Your Neutral is very unlikely to be the *same* potential as earth - close
certainly, but given they are both going to be low impedance paths there
is a good chance a substantial current will be able to flow between them
if connected.

If it is faulty, then passing 2.9A to earth would be a rather odd fault.
I expect you are simply measuring a large current flow between N and E


Well, sort of - the disconnected earth of the hob and the N of the
supply.

So that measurement is taken with the hob effectively not earthed at all
(i.e. the earth wire from the supply cable was not connected to anything)?

Correct - The hob was placed on the granite worktop, with only plastic parts
of the hob touching this worktop, so the appliance was not earthed for this
test.


If you diconnect the hob and measure the resistance between L & E and E &
N and L & N (with all the switches/controls on the hob turned off), what
readings do you get?

Not Quite as simple as this, as it has electronic touch controls, not knobs
or switched - however the readings a -

L-E - Starts at about 11Meg - then rapidly climbs to infinity
N-E - Ditto above
L-N - Starts at about 4.5meg, and settles at about 1.4meg


Sparks...

On Fri, 22 Apr 2005 12:21:05 +0100, "Fred" wrote:


"Peter Parry" wrote in message
.. .
On Thu, 21 Apr 2005 23:38:23 +0100, "Sparks" wrote:

If I measure the voltage, it is about 120v

Which is exactly as you would expect from a noise filtered device.

2.9A reactive current sounds far too much.

It is, I'm not altogether sure what is being measured here but a
current reading from chassis earth to mains earth is what is needed.
I'd expect it to be a few mA.

In reality the earth will be
close to neutral potential so the measurement is a fair one in this case.

He is measuring voltage from the floating hob chassis with the earth
disconnected. The noise filter capacitors will put the chassis at
half mains voltage in this situation.

You wouldn't allow the earth to run at 120V would you?

That's why you treat mains circuits with high protective circuit
currents carefully and take particular care with earth integrity.



--
Peter Parry.
http://www.wpp.ltd.uk/

On Fri, 22 Apr 2005 14:04:21 +0100, "Sparks" wrote:

"Peter Parry" wrote

Induction hobs use high frequency and high current in the induction
coils. The switching electronics produce a lot of RF interference
which needs to be filtered to stop it getting back into the mains
supply.

But this is happening with the hob on standby.

The filter is on the input - it is in circuit all the time unless the
hob isolator is open.

If I measure the current between its earth wire and Neutral, I am getting
about 2.9A

If you were really getting 2.9A flowing at 120V there is 350W being
dissipated somewhere in the hob all the time and you would feel the
heat generated without any problem at all.

The fact that the neutral to isolated chassis voltage is 120V
suggests the noise filter is serviceable and there is no marked
extraneous conduction path. The RCD is not always tripping
immediately which means there certainly isn't 2.9A or anything
remotely like it flowing all the time.

Can you connect the hob to a non-RCD circuit temporarily and measure
the current between the hob earth and mains earth?

What is the voltage between neutral and earth at the hob connector?

Connecting lots of things to a single 30mA RCD is a recipe for
nuisance tripping. You seem to have somewhat of an attachment to
RCD's so if you really want one connect the hob to a non-RCD circuit
and put a dedicated RCD on the hob alone.

So I will then get the 2.9A leaking to earth, 24/7 - Do I really want this?

Why would you have this? The hobs dedicated RCD would trip
immediately in this situation.



--
Peter Parry.
http://www.wpp.ltd.uk/

"Sparks" wrote in message
.. .
Just checking it's not just me, but I think my new induction hob is faulty!!

It is connected to the RCD consumer unit (Because the cable that feeds it
was feeding the oven, and was already connected there before you ask!)



What brand of Hob?

I have a Bosch induction hob which failed when new. When the Bosch engineer was
working on the upside down hob to repair it he got a shock touching some part
you would not expect to be live. We measured the voltage with his digital
voltmeter and got a reading of about 80v (or so). In normal operation the metal
bottom of the hob would have earthed the internal part that he touched. Not
having an RCD I live in happy ignorance of any currents that may be leaking to
earth.

--

Michael Chare

On Fri, 22 Apr 2005 15:50:09 +0100, "Michael Chare"
wrote:


What brand of Hob?

I have a Bosch induction hob which failed when new. When the Bosch engineer was
working on the upside down hob to repair it he got a shock touching some part
you would not expect to be live. We measured the voltage with his digital
voltmeter and got a reading of about 80v (or so). In normal operation the metal
bottom of the hob would have earthed the internal part that he touched. Not
having an RCD I live in happy ignorance of any currents that may be leaking to
earth.

All induction hobs (and any other piece of equipment using the usual
noise filters) will put "earthed" components at half mains potential
if the earth is disconnected. This is not a fault but a perfectly
normal characteristic (and why Sect 607 earthing should be considered
in houses). The current will not be enough to kill or directly cause
injury but can cause indirect harm by causing you to fall off
ladders, drop things etc.


--
Peter Parry.
http://www.wpp.ltd.uk/

Sparks wrote:


I tried disconnecting it's earth (and keeping away from it!), and the
tripping stopped
If I measure the current from it's earth wire to earth, the RCD trips
(no surprise there then!)

If I measure the current between its earth wire and Neutral, I am
getting about 2.9A
If I measure the voltage, it is about 120v

There've been quite a few replies explaining that this measurement isn't
valid, or at least is irrelevant to the problem, but I am note entirely
clear on why. My understanding of the test that you are performing is as
follows:

Appliance L connected to Supply L
Appliance N conencted to Supply N
Appliance E - not connected
Supply E - not connected

You then place the multimeter from the appliance earth to supply neutral,
and it read 2.9A. To me, this clearly doesn't sound healthy. In a healthy
appliance, the earth should isolated from both live and neutral (with the
exception of a high impedance link via the noise suppression caps), and
should hence not be able to supply any current to neutral.

The point that was being made is that if you place a multimeter between the
supply earth and neutral then significant current will flow. This is
clearly true, as N and E are held different potentials via a relatively low
impedance, depending on the earthing system. However, in Sparks' case, he
is not measuring current from supply earth to neutral, but from the
appliance earth to neutral, and the appliance earth should be isolated from
any potentials as he has disconnected the earth terminal. It is possible
that some form of supplementary bonding is in place, and introducing a
potential on the appliance earth, but this seems unlikely because his RCD
ceased to trip when the earth terminal was disonnected. Also, the fact that
the RCD doesn't trip, even when the multimeter is showing 2.9A flowing to
neutral, suggests that this current is being sourced from the RCD protected
supply.

If my understanding of the tests that have been performed is correct, then
this does sound clearly like a faulty device.


Chris Key

There've been quite a few replies explaining that this measurement isn't
valid, or at least is irrelevant to the problem, but I am note entirely
clear on why. My understanding of the test that you are performing is as
follows:

Appliance L connected to Supply L
Appliance N conencted to Supply N
Appliance E - not connected
Supply E - not connected

At last, someone who can see my point )

You then place the multimeter from the appliance earth to supply neutral,
and it read 2.9A. To me, this clearly doesn't sound healthy. In a
healthy appliance, the earth should isolated from both live and neutral
(with the exception of a high impedance link via the noise suppression
caps), and should hence not be able to supply any current to neutral.

....yeeessss!

The point that was being made is that if you place a multimeter between
the supply earth and neutral then significant current will flow. This is
clearly true, as N and E are held different potentials via a relatively
low impedance, depending on the earthing system. However, in Sparks'
case, he is not measuring current from supply earth to neutral, but from
the appliance earth to neutral, and the appliance earth should be isolated
from any potentials as he has disconnected the earth terminal. It is
possible that some form of supplementary bonding is in place, and
introducing a potential on the appliance earth, but this seems unlikely
because his RCD ceased to trip when the earth terminal was disonnected.
Also, the fact that the RCD doesn't trip, even when the multimeter is
showing 2.9A flowing to neutral, suggests that this current is being
sourced from the RCD protected supply.

If my understanding of the tests that have been performed is correct, then
this does sound clearly like a faulty device.

I eventually managed to get through to someone at Siemens who has a clue,
and it is going back!

Chris Key

On Fri, 22 Apr 2005 21:28:09 +0100, "Christopher Key"
wrote:


You then place the multimeter from the appliance earth to supply neutral,
and it read 2.9A. To me, this clearly doesn't sound healthy.

In most appliances I would agree but induction hobs have a heap of
electronics in there to produce the high frequency high current
needed to operate. I'm not altogether convinced the test is valid
hence the suggestion that an earth-earth test be carried out on a
non-RCD circuit.

There are a number of inconsistencies:-

If the leakage current really was consistently a few amps the RCD
would always trip immediately. It isn't. It is tripping after some
time and somewhat unpredictably - this points to a situation where
the total asymmetry in the house is hovering about the minimum trip
current.

The voltage is about half mains - this is exactly what you would
expect from a noise filtered circuit with a floating earth - any
leakage path would alter this reading.

In a healthy
appliance, the earth should isolated from both live and neutral (with the
exception of a high impedance link via the noise suppression caps), and
should hence not be able to supply any current to neutral.

I'd expect a few mA leakage through the filter.

If my understanding of the tests that have been performed is correct, then
this does sound clearly like a faulty device.

Except that the results of the tests and the reported fault symptoms
are mutually exclusive. With a permanent leakage of this level the
RCD would always trip immediately but apparently it isn't. The
voltage reading is also inconsistent with a single fault capable of
supplying this current.

--
Peter Parry.
http://www.wpp.ltd.uk/

Peter Parry wrote:
On Fri, 22 Apr 2005 21:28:09 +0100, "Christopher Key"
wrote:


You then place the multimeter from the appliance earth to supply
neutral, and it read 2.9A. To me, this clearly doesn't sound
healthy.

In most appliances I would agree but induction hobs have a heap of
electronics in there to produce the high frequency high current
needed to operate. I'm not altogether convinced the test is valid
hence the suggestion that an earth-earth test be carried out on a
non-RCD circuit.


I can certainly imagine the the high frequencies being leaked could confuse
a DMM, but can't see why the results would be any different if the leakage
was measured to earth and not neutral. Certainly no harm in trying if it's
possible though.

There are a number of inconsistencies:-

If the leakage current really was consistently a few amps the RCD
would always trip immediately. It isn't. It is tripping after some
time and somewhat unpredictably - this points to a situation where
the total asymmetry in the house is hovering about the minimum trip
current.

The voltage is about half mains - this is exactly what you would
expect from a noise filtered circuit with a floating earth - any
leakage path would alter this reading.

In a healthy
appliance, the earth should isolated from both live and neutral
(with the exception of a high impedance link via the noise
suppression caps), and should hence not be able to supply any
current to neutral.

I'd expect a few mA leakage through the filter.

If my understanding of the tests that have been performed is
correct, then this does sound clearly like a faulty device.

Except that the results of the tests and the reported fault symptoms
are mutually exclusive. With a permanent leakage of this level the
RCD would always trip immediately but apparently it isn't. The
voltage reading is also inconsistent with a single fault capable of
supplying this current.


I take your points onboard, but still maintain my original argument that the
multimeter was correctly connected to measure any leakage to earth. Whether
it was actually doing that, or reporting spurious values as a result of
induced noise, or an odd waveform is another matter. Would be interesting
to see what happens with a moving coil meter.

As you say, there is no way it could be leaking 3A to earth consistently,
and only occasionally trip the RCD. It is conceivable that it is an
intermittent fault however, as reading the OP, I got the impression that
typically the RCD tripped instantly, but occasionally was ok for a while.
This could be consistent with an intermittent short internally, that was
open when the 120V was read, and whilst the RCD wasn't tripping. That does
seem a little far fetched though.

There are combinations of whole house faults that could cause the behaviour
described, but they are so contrived that I doubt they could be created even
if one tried!

It seems like the hob is being replaced however, so I shall be interested to
see the behaviour of the replacement.

Chris Key

Sparks wrote:

At last, someone who can see my point )

Yep, you've cleared up that possible misunderstanding.

I don't think anyone has asked what sort of meter you're using. At the
risk of raising a cry of "grandmother, eggs, sucking," are you sure that
the reading was really 2.9 A and not perhaps 2.9 mA? I'm thinking that
some of the Fluke DMMs just display a number on current ranges, and
whether it means amps, milliamps or microamps depends on which socket
you've plugged the red test lead into.

Even 2.9 mA's a bit on the high side, but much more feasible than having
nearly 3 A flowing in the earth wire.

What happens if you use a 12V 50W bulb in place of the meter? At 2.9 A
it should light up nicely.

I eventually managed to get through to someone at Siemens who has a clue,
and it is going back!

Perhaps it's too late and we'll never know...

--
Andy

"Peter Parry" wrote in message
news
On Fri, 22 Apr 2005 15:50:09 +0100, "Michael Chare"
wrote:


What brand of Hob?

I have a Bosch induction hob which failed when new. When the Bosch engineer
was
working on the upside down hob to repair it he got a shock touching some part
you would not expect to be live. We measured the voltage with his digital
voltmeter and got a reading of about 80v (or so). In normal operation the
metal
bottom of the hob would have earthed the internal part that he touched. Not
having an RCD I live in happy ignorance of any currents that may be leaking
to
earth.

All induction hobs (and any other piece of equipment using the usual
noise filters) will put "earthed" components at half mains potential
if the earth is disconnected. This is not a fault but a perfectly
normal characteristic (and why Sect 607 earthing should be considered
in houses). The current will not be enough to kill or directly cause
injury but can cause indirect harm by causing you to fall off
ladders, drop things etc.

Yes, in this case the appliance was correctly earthed. The problem for the
repair man was that the bottom plate formed part of the earth circuit. A better
design would have ensured that the internal compnents were earthed even with the
bottom plate removed.
(Atleast that is how it appears, I have not studied it in detail)

--

Michael Chare

On Sat, 23 Apr 2005 11:42:59 +0100, Andy Wade
wrote:


Even 2.9 mA's a bit on the high side, but much more feasible than having
nearly 3 A flowing in the earth wire.

2.9mA is about right for a noise filter (usually 2-3.5mA)

--
Peter Parry.
http://www.wpp.ltd.uk/

In article ,
Peter Parry writes:
On Sat, 23 Apr 2005 11:42:59 +0100, Andy Wade
wrote:


Even 2.9 mA's a bit on the high side, but much more feasible than having
nearly 3 A flowing in the earth wire.

2.9mA is about right for a noise filter (usually 2-3.5mA)

For Class I IT appliances (ie. computer), max permitted is 0.75 mA.
They are normally about half this though. I think this limit applies
to most types of Class I appliances which plug in to a standard socket
outlet, but they are all defined in different EN documents, so it
would take some research (and considerable expense) to get a full
list of figures. Wired-in appliances are probably allowed higher
leakage.

--
Andrew Gabriel