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.
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
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
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?....
On Sat, 11 Sep 2004 23:26:32 +0100, tony sayer strung together this:
That may be but the instantaneous current that flows immediately prior to a circuit protective device operating is many thousands of amps.
In article , Lurch writes
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
In article , Dave Stanton writes
Well they are always looking to disprove this and that. Such as is progress made. When I see him again I'll ask:)
On Sun, 12 Sep 2004 14:48:49 +0100, tony sayer strung together this:
Yes, he must know everything. Ask him where that £5 note is that I lost last week, it'd be most helpful.
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.
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.
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, snipped-for-privacy@btopenworld.com (Ken Knott) strung together this:
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)!
Wow, that's high -- overhead or underground distribution? Have you tried complaining about undervoltage?
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.
Nope, as I'm not suffering any ill effects from it.
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.
Oops, overhead.
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