I have purchased a CED immersion heater timer model IMT7E. This timer
does not appear to have an Earth terminal so I am not sure what to do
with the Earth wire that comes out of the heating element? Can anyone
advise me? Thanks.

On 11 Sep 2004 12:18:43 -0700, (Ken Knott)
wrote:

hi ken,
it is normal when connecting a timer to 'break' into the flex running
from the immersion heater to the time switch, the earth will continue
through, this probably will be a terminal on the timer witch box.
if there is not a connection then you need to make one with a 20Amp
connector, this is very important.
regards
bob

I have purchased a CED immersion heater timer model IMT7E. This timer
does not appear to have an Earth terminal so I am not sure what to do
with the Earth wire that comes out of the heating element? Can anyone
advise me? Thanks.

Ken Knott wrote:

I have purchased a CED immersion heater timer model IMT7E. This timer
does not appear to have an Earth terminal so I am not sure what to do
with the Earth wire that comes out of the heating element? Can anyone
advise me? Thanks.

The timer will be either single-pole, switching only the live, or
double-pole, switching both live and nuetral. The earth should NEVER be
switched; you join up the incoming and outgoing earths in a handy
terminal in the enclosure wot the timer switch is in (either a terminal
provided in the backbox, which all backboxes sold in the last 15+ years
should have, or in extremis a single-way bit of terminal block).

If you're not sure about this, you may want to do some more reading and
finding-out about electrics before tackling such jobs; 240V with
kilo-amps of Prospective Fault Current is no joke...

Cheers, Stefek

If you're not sure about this, you may want to do some more reading and
finding-out about electrics before tackling such jobs; 240V with
kilo-amps of Prospective Fault Current is no joke...

Cheers, Stefek


Now correct moi if I'm wrong but didn't someone say on here the other
week that substations were on fuses of 300 amps or thereabouts?....
--
Tony Sayer

On Sat, 11 Sep 2004 23:26:32 +0100, tony sayer
strung together this:

If you're not sure about this, you may want to do some more reading and
finding-out about electrics before tackling such jobs; 240V with
kilo-amps of Prospective Fault Current is no joke...

Now correct moi if I'm wrong but didn't someone say on here the other
week that substations were on fuses of 300 amps or thereabouts?....

That may be but the instantaneous current that flows immediately prior
to a circuit protective device operating is many thousands of amps.
--

SJW
A.C.S. Ltd

In article , Lurch
writes
On Sat, 11 Sep 2004 23:26:32 +0100, tony sayer
strung together this:

If you're not sure about this, you may want to do some more reading and
finding-out about electrics before tackling such jobs; 240V with
kilo-amps of Prospective Fault Current is no joke...

Now correct moi if I'm wrong but didn't someone say on here the other
week that substations were on fuses of 300 amps or thereabouts?....

That may be but the instantaneous current that flows immediately prior
to a circuit protective device operating is many thousands of amps.

Well a uni researcher I was talking to a while ago didn't seem to think
it was quite like that "all" the time, but this was a while ago now....
--
Tony Sayer

That may be but the instantaneous current that flows immediately prior
to a circuit protective device operating is many thousands of amps.

Well a uni researcher I was talking to a while ago didn't seem to think
it was quite like that "all" the time, but this was a while ago now....

What did he think then, the laws of physics change with the seasons...

Dave

--

Some people use windows, others have a life.

In article , Dave Stanton
writes

That may be but the instantaneous current that flows immediately prior
to a circuit protective device operating is many thousands of amps.

Well a uni researcher I was talking to a while ago didn't seem to think
it was quite like that "all" the time, but this was a while ago now....

What did he think then, the laws of physics change with the seasons...

Dave


Well they are always looking to disprove this and that. Such as is
progress made. When I see him again I'll ask
--
Tony Sayer

On Sun, 12 Sep 2004 14:48:49 +0100, tony sayer
strung together this:

When I see him again I'll ask

Yes, he must know everything. Ask him where that £5 note is that I
lost last week, it'd be most helpful.
--

SJW
A.C.S. Ltd

tony sayer wrote:

['Lurch' wrote:]
That may be but the instantaneous current that flows immediately prior
to a circuit protective device operating is many thousands of amps.

Well they are always looking to disprove this and that. Such as is
progress made. When I see him again I'll ask

It's very simple you know. The short circuit current depends on the
voltage (EMF) of the source and the impedance of the circuit between the
distribution transformer [1] and the point where you have elected to
apply a short-circuit fault. I = V / Z.

For single phase mains the EMF is nominally 230 V, 240-250 V in
practice. The impedance is highly dependent on the length of cable
between the transformer and the fault.

For public mains the supply industry will tell you that the maximum
prospective fault current at the supply terminals is 16 kA [2] -
implying an impedance of about 15 milliohms - most of which is the
leakage reactance of the transformer. Such high fault levels are
actually pretty rare and only arise when you are very close to the
substation.

For the other end of the scale we can use the maximum external earth
fault loop impedance value quoted by the industry for PME supplies.
This is 0.35 ohm - implying a minimum fault level of about 700 A.
Again, this is the relatively uncommon other end of the scale - where
you are on the end of a long line out in the sticks.

None of this depends on the rating of any fuses in the way, except to
the extent that each fuse adds a tiny bit of resistance to the circuit.
Obviously, lower rated fuses will blow more quickly at any
particular fault level, but the full current will flow until the fusible
elements have melted and the ensuing arc has been quenched. An
exception can arise at high fault level where the fuse can clear the
fault within less than half a mains cycle, so that peak value of the
current waveform is never reached; the fuse is then said to exhibit
'current limiting' behaviour.


[1] This ignores the rest of the network on the supply side of the
transformer, which has near-enough negligible impedance for the matter
under consideration.

[2] Except in parts of the old LEB area where larger transformers and
cables may be in use.

--
Andy

Andy Wade wrote:
tony sayer wrote:

['Lurch' wrote:]
That may be but the instantaneous current that flows immediately prior
to a circuit protective device operating is many thousands of amps.

Well they are always looking to disprove this and that. Such as is
progress made. When I see him again I'll ask

It's very simple you know. The short circuit current depends on the
voltage (EMF) of the source and the impedance of the circuit between the
distribution transformer [1] and the point where you have elected to
apply a short-circuit fault. I = V / Z.
snip
For the other end of the scale we can use the maximum external earth
fault loop impedance value quoted by the industry for PME supplies.
This is 0.35 ohm - implying a minimum fault level of about 700 A.
Again, this is the relatively uncommon other end of the scale - where
you are on the end of a long line out in the sticks.

For reference, I'm in the middle of a village, about 100m from the nearest
substation, and get about 0.5 ohms.

If you've got a voltmeter and enough clue to connect it to the mains without
killing yourself, this is easy to measure.
Take a known load, for example the immersion heater or an electric shower.
Measure the voltage with this off and on.
You have to do this a few times, as the mains voltage varies a bit normally.

This gives you a measure of the impedance from the busbars of your
consumer unit back to the nuclear/gas/solar/wind/... power station.

Meausuring and using a load on the same circuit isn't a good idea as this
adds that resistance in.

Andy Wade wrote in message ...
tony sayer wrote:

['Lurch' wrote:]
That may be but the instantaneous current that flows immediately prior
to a circuit protective device operating is many thousands of amps.

Well they are always looking to disprove this and that. Such as is
progress made. When I see him again I'll ask

It's very simple you know. The short circuit current depends on the
voltage (EMF) of the source and the impedance of the circuit between the
distribution transformer [1] and the point where you have elected to
apply a short-circuit fault. I = V / Z.

For single phase mains the EMF is nominally 230 V, 240-250 V in
practice. The impedance is highly dependent on the length of cable
between the transformer and the fault.

For public mains the supply industry will tell you that the maximum
prospective fault current at the supply terminals is 16 kA [2] -
implying an impedance of about 15 milliohms - most of which is the
leakage reactance of the transformer. Such high fault levels are
actually pretty rare and only arise when you are very close to the
substation.

For the other end of the scale we can use the maximum external earth
fault loop impedance value quoted by the industry for PME supplies.
This is 0.35 ohm - implying a minimum fault level of about 700 A.
Again, this is the relatively uncommon other end of the scale - where
you are on the end of a long line out in the sticks.

None of this depends on the rating of any fuses in the way, except to
the extent that each fuse adds a tiny bit of resistance to the circuit.
Obviously, lower rated fuses will blow more quickly at any
particular fault level, but the full current will flow until the fusible
elements have melted and the ensuing arc has been quenched. An
exception can arise at high fault level where the fuse can clear the
fault within less than half a mains cycle, so that peak value of the
current waveform is never reached; the fuse is then said to exhibit
'current limiting' behaviour.


[1] This ignores the rest of the network on the supply side of the
transformer, which has near-enough negligible impedance for the matter
under consideration.

[2] Except in parts of the old LEB area where larger transformers and
cables may be in use.

Well, thanks for all this guys! I am actually reasonably OK with
mains electrics, its just that not having an earth terminal on the
timer rather threw me. I've now run an Earth wire from the earth
terminal on the immersion heater to the earth terminal in the switch
that supplies the mains to the timer. Can anyone see anything wrong
with this arrangement?

On 12 Sep 2004 11:46:44 -0700, (Ken Knott)
strung together this:

Well, thanks for all this guys! I am actually reasonably OK with
mains electrics, its just that not having an earth terminal on the
timer rather threw me. I've now run an Earth wire from the earth
terminal on the immersion heater to the earth terminal in the switch
that supplies the mains to the timer. Can anyone see anything wrong
with this arrangement?

Nope, as long as the main earth at the switch is connected to the
immersion heater earth terminal then all is well, (as far as earthed
immersion heaters go)!
--

SJW
A.C.S. Ltd

Ian Stirling wrote:

For reference, I'm in the middle of a village, about 100m from the nearest
substation, and get about 0.5 ohms.

Wow, that's high -- overhead or underground distribution? Have you
tried complaining about undervoltage?

If you've got a voltmeter and enough clue to connect it to the mains without
killing yourself, this is easy to measure.
[...]
Meausuring and using a load on the same circuit isn't a good idea as this
adds that resistance in.

It's absolutely essential to measure on a different circuit to the one
you're loading - or straight on the CU busbars, using fused test leads
of course - otherwise the results you get are meaningless.

--
Andy

Andy Wade wrote:
Ian Stirling wrote:

For reference, I'm in the middle of a village, about 100m from the nearest
substation, and get about 0.5 ohms.

Wow, that's high -- overhead or underground distribution? Have you
tried complaining about undervoltage?

Nope, as I'm not suffering any ill effects from it.

If you've got a voltmeter and enough clue to connect it to the mains without
killing yourself, this is easy to measure.
[...]
Meausuring and using a load on the same circuit isn't a good idea as this
adds that resistance in.

It's absolutely essential to measure on a different circuit to the one
you're loading - or straight on the CU busbars, using fused test leads
of course - otherwise the results you get are meaningless.

True.
Well, it is theoretically possible to compensate, but in practice you'r
either going to need exact lengths of wiring, or some means of measuring,
which will involve another circuit.
Otherwise you add the resistance of the ring back to the fusebox and the
fuse/breaker.

Andy Wade wrote:
Ian Stirling wrote:

For reference, I'm in the middle of a village, about 100m from the nearest
substation, and get about 0.5 ohms.

Wow, that's high -- overhead or underground distribution? Have you
tried complaining about undervoltage?
snip
Oops, overhead.