Blimey! Thanks for the explanation Stefek.

Could you please explain why a motor which runs at a lower that nominated
voltage, draws more current to keep the power output the same?

Mike.

On 17 Jan 2004 23:09:25 GMT, wrote:

massive snip

Thanks Stefek, for your comprehensive reply.

Just to fill in a couple of blanks, the cable (10mm2), the RCD and the
40amp MCB were all installed at the same time as the shower. There was no
previous electric shower. There is definately nothing else on the same
circuit. The total length of the cable run is about 6 metres.

I will get my brother to test his RCD. Is the built in test button a
sufficiently good test to rule out a problem with the RCD?

I believe the shower is still within guarantee, so if the RCD works and
all the connections look sound, we'll get someone out to have a look at
it.

Cheers
Simon

I recently went around this exact loop when installing a shower.

It was a similarly rated Triton model to that whose spec was mentioned by
Ano poster(s) - i.e. 8.7KW at 230V, 9.5 KW at 240 V.

Anyway, when rating the cables etc. I thought I'd better measure the local
voltage. Turned out to be 248 Volts under moderate load, which implied a
draw of a shade under 41 Amps, IIRC, for the shower. (Making the assumption
of a linear resistance change with increasing voltage - not 100% correct,
but a reasonable approximation here).

Should also say the load of the shower would pull the actual supply voltage
down a bit, so reducing the current actually drawn... never checked how
much.

Anyway, as the Triton installation guide said "40/45 Amp MCB" for this
nominal 9.5 KW unit, I fitted the latter.

Given my problem finding a **competent** electrician, I'd think it would be
worth you checking the local voltage **but only if you feel you can do this
safely**, just to rule this in/out as a possibility.

BUT, as has been correctly stated, MCB's don't just trip out as soon as the
rated current is exceeded - typically a so-called type B will allow double
the current for 60 seconds or more. How fast does it trip ??


"coherers"


"smb" wrote in message
...
On 17 Jan 2004 23:09:25 GMT,
wrote:

massive snip

Thanks Stefek, for your comprehensive reply.

Just to fill in a couple of blanks, the cable (10mm2), the RCD and the
40amp MCB were all installed at the same time as the shower. There was no
previous electric shower. There is definately nothing else on the same
circuit. The total length of the cable run is about 6 metres.

I will get my brother to test his RCD. Is the built in test button a
sufficiently good test to rule out a problem with the RCD?

I believe the shower is still within guarantee, so if the RCD works and
all the connections look sound, we'll get someone out to have a look at
it.

Cheers
Simon

On Sun, 18 Jan 2004 18:55:56 GMT, Coherers
wrote:
I recently went around this exact loop when installing a shower.

It was a similarly rated Triton model to that whose spec was mentioned by
Ano poster(s) - i.e. 8.7KW at 230V, 9.5 KW at 240 V.

Anyway, when rating the cables etc. I thought I'd better measure the local
voltage. Turned out to be 248 Volts under moderate load, which implied a
draw of a shade under 41 Amps, IIRC, for the shower. (Making the assumption
of a linear resistance change with increasing voltage - not 100% correct,
but a reasonable approximation here).

Should also say the load of the shower would pull the actual supply voltage
down a bit, so reducing the current actually drawn... never checked how
much.

Anyway, as the Triton installation guide said "40/45 Amp MCB" for this
nominal 9.5 KW unit, I fitted the latter.

Given my problem finding a **competent** electrician, I'd think it would be
worth you checking the local voltage **but only if you feel you can do this
safely**, just to rule this in/out as a possibility.

BUT, as has been correctly stated, MCB's don't just trip out as soon as the
rated current is exceeded - typically a so-called type B will allow double
the current for 60 seconds or more. How fast does it trip ??


"coherers"


I've got a multimeter that can handle upto 700 volts ac, so I'll take it
round there and get a read for the voltage.

My brother gets up first in the morning and usually has a shower OK
without it tripping. When his wife get in the shower a bit later on, it
often trips after 5 min or so. Not always, but increasingly often.

The RCD has been tested, using the built in test button, and appears to
work correctly.


"smb" wrote in message
...
On 17 Jan 2004 23:09:25 GMT,
wrote:

massive snip

Thanks Stefek, for your comprehensive reply.

Just to fill in a couple of blanks, the cable (10mm2), the RCD and the
40amp MCB were all installed at the same time as the shower. There was no
previous electric shower. There is definately nothing else on the same
circuit. The total length of the cable run is about 6 metres.

I will get my brother to test his RCD. Is the built in test button a
sufficiently good test to rule out a problem with the RCD?

I believe the shower is still within guarantee, so if the RCD works and
all the connections look sound, we'll get someone out to have a look at
it.

Cheers
Simon

"Mike G" wrote in message
...
Blimey! Thanks for the explanation Stefek.

Could you please explain why a motor which runs at a lower that nominated
voltage, draws more current to keep the power output the same?

very good explanation here
http://www.ecmweb.com/ar/electric_highs_lows_motor/

On Sun, 18 Jan 2004 19:30:51 UTC, "Chris Oates" none wrote:


"Mike G" wrote in message
...
Blimey! Thanks for the explanation Stefek.

Could you please explain why a motor which runs at a lower that nominated
voltage, draws more current to keep the power output the same?

very good explanation here
http://www.ecmweb.com/ar/electric_highs_lows_motor/

It doesn't really explain why a motor doesn't behave like a resistive
load; it states 'it does this' but glosses over 'why'.


--
Bob Eager
rde at tavi.co.uk
PC Server 325*4; PS/2s 9585, 8595, 9595*2, 8580*3,
P70...

Best to test the voltage when the shower is on if you can ( But your test
points not nearby!)

Reading graphs for various MCBs, I figure it would need to be drawing *at
least* 50A for 5 minutes to trip a "B" or "C", which seems unlikely.
However, really depends on the exact characteristics of the type in use. And
it could have wandered off spec, hence it starting to trip where it never
did before.

If it is drawing over 40 A, and the shower manufacturer allows for it,
replacing the MCB (£6 or £7 IIRC) could be a fast and cheap diagnostic/fix.
However, as I recall not all consumer units brands take the 45A flavour and
the 50 A is a definite no-no....

However, following up the other posters' comments on safety, I would
seriously recommend no-one uses the shower for its intended purpose until
you have got to the bottom of this one.....


"smb" wrote in message
...
On Sun, 18 Jan 2004 18:55:56 GMT, Coherers
wrote:
I recently went around this exact loop when installing a shower.

It was a similarly rated Triton model to that whose spec was mentioned
by
Ano poster(s) - i.e. 8.7KW at 230V, 9.5 KW at 240 V.

Anyway, when rating the cables etc. I thought I'd better measure the
local
voltage. Turned out to be 248 Volts under moderate load, which implied a
draw of a shade under 41 Amps, IIRC, for the shower. (Making the
assumption
of a linear resistance change with increasing voltage - not 100%
correct,
but a reasonable approximation here).

Should also say the load of the shower would pull the actual supply
voltage
down a bit, so reducing the current actually drawn... never checked how
much.

Anyway, as the Triton installation guide said "40/45 Amp MCB" for this
nominal 9.5 KW unit, I fitted the latter.

Given my problem finding a **competent** electrician, I'd think it would
be
worth you checking the local voltage **but only if you feel you can do
this
safely**, just to rule this in/out as a possibility.

BUT, as has been correctly stated, MCB's don't just trip out as soon as
the
rated current is exceeded - typically a so-called type B will allow
double
the current for 60 seconds or more. How fast does it trip ??


"coherers"


I've got a multimeter that can handle upto 700 volts ac, so I'll take it
round there and get a read for the voltage.

My brother gets up first in the morning and usually has a shower OK
without it tripping. When his wife get in the shower a bit later on, it
often trips after 5 min or so. Not always, but increasingly often.

The RCD has been tested, using the built in test button, and appears to
work correctly.


"smb" wrote in message
...
On 17 Jan 2004 23:09:25 GMT,
wrote:

massive snip

Thanks Stefek, for your comprehensive reply.

Just to fill in a couple of blanks, the cable (10mm2), the RCD and the
40amp MCB were all installed at the same time as the shower. There was
no
previous electric shower. There is definately nothing else on the same
circuit. The total length of the cable run is about 6 metres.

I will get my brother to test his RCD. Is the built in test button a
sufficiently good test to rule out a problem with the RCD?

I believe the shower is still within guarantee, so if the RCD works and
all the connections look sound, we'll get someone out to have a look at
it.

Cheers
Simon

"Coherers" wrote in message
...
Best to test the voltage when the shower is on if you can ( But your test
points not nearby!)

Reading graphs for various MCBs, I figure it would need to be drawing *at
least* 50A for 5 minutes to trip a "B" or "C", which seems unlikely.
However, really depends on the exact characteristics of the type in use.
And
it could have wandered off spec, hence it starting to trip where it never
did before.

If it is drawing over 40 A, and the shower manufacturer allows for it,
replacing the MCB (£6 or £7 IIRC) could be a fast and cheap
diagnostic/fix.
However, as I recall not all consumer units brands take the 45A flavour
and
the 50 A is a definite no-no....

'wandering' is what mine did - I brought home the clamp meter
and found it tripping after 4-5 minutes at 36A

However, following up the other posters' comments on safety, I would
seriously recommend no-one uses the shower for its intended purpose until
you have got to the bottom of this one.....

ripper wrote:
Alistair
You are trying to find current from two known quantities, voltage and
power. Power remains constant, voltage varies.
P=V*I
P=9500 Watts
Voltage = 230
Therefore I = P/V which is 9500/230 = 41.3 Amps the shower will take
at 9.5 kW.
[SNIP a load of tosh]

Wrong the power will go up with the suare of the voltage. e.g. voltage goes
up 10% poer taken will go approx 20%.
Yes, the heater resistance will change with temperature but not to a great
extent.

smb wrote in message ...

My brother gets up first in the morning and usually has a shower OK
without it tripping. When his wife get in the shower a bit later on, it
often trips after 5 min or so. Not always, but increasingly often.

That means tha the MCB is tripping thermally, due to a sustained
slight overload, rather than magnetically, due to a heavy overload.

Serious question: How long does your brother take in the shower? If
he is one of those people who takes quite a while, this could explain
the problem.

And how soon after him does his wife shower?

On 18 Jan 2004 17:12:22 -0800, Chippy wrote:
smb wrote in message ...

My brother gets up first in the morning and usually has a shower OK
without it tripping. When his wife get in the shower a bit later on, it
often trips after 5 min or so. Not always, but increasingly often.

That means tha the MCB is tripping thermally, due to a sustained
slight overload, rather than magnetically, due to a heavy overload.

Serious question: How long does your brother take in the shower? If
he is one of those people who takes quite a while, this could explain
the problem.

And how soon after him does his wife shower?

I don't know their showering habits well enought to give an answer. I
will have another conversation with them to find out. Thing is, they have
had the shower for several months, and their morning routine has not
changed recently, so why should it start tripping and why should it trip
more often as time goes on.

Simon

On Sun, 18 Jan 2004 21:03:26 +0000, Bob Eager wrote:

On Sun, 18 Jan 2004 19:30:51 UTC, "Chris Oates" none wrote:


"Mike G" wrote in message
...
Blimey! Thanks for the explanation Stefek.

Could you please explain why a motor which runs at a lower that nominated
voltage, draws more current to keep the power output the same?

very good explanation here
http://www.ecmweb.com/ar/electric_highs_lows_motor/

It doesn't really explain why a motor doesn't behave like a resistive
load; it states 'it does this' but glosses over 'why'.

To be fair, I think the article assumes a backrground knowledge of electrical
engineering. Where term like "power factor" and "magnetic saturation"
would be concepts the reader is expected to know about beforehand.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

smb wrote in message news:9zVOb.7010$

I don't know their showering habits well enought to give an answer. I
will have another conversation with them to find out. Thing is, they have
had the shower for several months, and their morning routine has not
changed recently, so why should it start tripping and why should it trip
more often as time goes on.

I can only suggest that the thermal part of the MCB is becoming
'tired' due to repeated tripping and/or lengthy periods of being
heated to an 'almost trip' state.

If the usage cycle of the shower is such that it normally has almost
but not quit reached a trip state, it wouldn't take much cahnge t
cause the problems that you have described.

A few minutes extra use each day, your sister-in-law's shower
following your brother's more closely, a slight change in line voltage
- any ro all of these could affect the situation.

One simple test I'd suggest - turn on the shower, leave it running,
and see how long it takes to trip.

In uk.d-i-y, smb wrote:
On 17 Jan 2004 23:09:25 GMT, wrote:

massive snip

Thanks Stefek, for your comprehensive reply.

Cheers. Radio silence from me is because I'm in the US for a week; just
chewing through uk.d-i-y backlog while considering wording of major document
I need to start on tonight ;-)

Just to fill in a couple of blanks, the cable (10mm2), the RCD and the
40amp MCB were all installed at the same time as the shower. There was no
previous electric shower. There is definately nothing else on the same
circuit. The total length of the cable run is about 6 metres.

OK, so the more worrying possibilities become less likely (phew!) It's
possible then that the positioning of the MCB between other loaded MCBs
is the problem - they're warming one another up and making the shower one
trip. But that's not nearly as likely as a fault in the shower, methinks...

I will get my brother to test his RCD. Is the built in test button a
sufficiently good test to rule out a problem with the RCD?

Sadly not: if the test button doesn't make the RCD pop, the RCD *is* faulty
for sure. But if it does pop, the RCD could still be out of spec: too
sensitive, not sensitive enough, take too long to pop (not as if you can
reliably press the test button for exactly 20 milliseconds, is it!). But
in your brother's case it's an individual MCB which is popping, not the
RCD, right? To "properly" test an RCD needs a "proper" RCD tester, which
full-time electricians can spread the 700-quid cost of over years of work,
but which thee and me aren't going to shell out on! It'd make a good
school *design* project, though, using suitable resistors to make a 10mA,
15mA, and 30mA load, and a gravity-dependant timing element such as a steel
ball running down an inclined channel with a conductive section: lots
of good simple applied maths working out how to do various short timing
intervals. Course, Health-n-Safety being wot it is, you couldn't ever
*build* one with the kids :-(

I believe the shower is still within guarantee, so if the RCD works and
all the connections look sound, we'll get someone out to have a look at
it.

Hope you/he get it all sorted... Stefek

"Bob Eager" wrote in message ...
On Sat, 17 Jan 2004 19:28:23 UTC, "ripper"
wrote:

You are trying to find current from two known quantities, voltage and power.
Power remains constant, voltage varies.

No, it doesn't.

And if you use 110 Volts, the amps now increase to 86.4 Amps

LOL!

So if I apply a 1.5 volts from a Duracell AA cell, I can get about 6300
amps? Lovely!

Better yet, consider the situation if you were to ever cut the supply!
to maintain the somewhat dotty theory that the current will vary to
keep power constant as the voltage changes (I = P/V where P is
constant) the current I must become infinite!!!

Conclusions, either:
a) When you turn off a shower (or a light bulb) the power
dissipated remains constant and the current is infinite.
OR
b) The constant power idea is tosh and it is the resistance that
stays constant.


Please avert your eyes if you are offended by someone shamelessly
waving their qualifications about - I don't like to do it but it seems
reasonable at this point... I have a masters degree in electrical
engineering, came top of my class, was awarded a prize by the
Institution of Electrical Engineering (the IEE) and my grasp of
electrical theory leads me to opt for answer "b". Anyone think they
have any better ideas?

On Fri, 23 Jan 2004 10:27:25 +0000, Matt Beard wrote:

"Bob Eager" wrote in message ...
On Sat, 17 Jan 2004 19:28:23 UTC, "ripper"
wrote:

You are trying to find current from two known quantities, voltage and power.
Power remains constant, voltage varies.

No, it doesn't.

And if you use 110 Volts, the amps now increase to 86.4 Amps

LOL!

So if I apply a 1.5 volts from a Duracell AA cell, I can get about 6300
amps? Lovely!

Better yet, consider the situation if you were to ever cut the supply!
to maintain the somewhat dotty theory that the current will vary to
keep power constant as the voltage changes (I = P/V where P is
constant) the current I must become infinite!!!

Conclusions, either:
a) When you turn off a shower (or a light bulb) the power
dissipated remains constant and the current is infinite.
OR
b) The constant power idea is tosh and it is the resistance that
stays constant.


Please avert your eyes if you are offended by someone shamelessly
waving their qualifications about - I don't like to do it but it seems
reasonable at this point... I have a masters degree in electrical
engineering, came top of my class, was awarded a prize by the
Institution of Electrical Engineering (the IEE) and my grasp of
electrical theory leads me to opt for answer "b". Anyone think they
have any better ideas?

Yes. The resistance remains very nearly constant.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html