More Elect Questions

I posted a couple of questions on a previous thread and many thanks to all that replied. But as always answers lead to more questions. The two questions I asked about were:

  1. Prospective fault current
  2. External loop impedance

I now find myself asking WHY "do I / should I" need to know. In a domestic situation where I am buying off the shelf BS certified products, what would I do differently by knowing/having this information. I don't see Hager selling Consumer Unit products specifically for use on TN-S as oppossed to TN-S-C

Maybe I missing some fundemental point relating to the calculation of fault currents and hence MCB ratings.

Reply to
ac1951
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DOH.... Think I've just realised the answer. I would need to know this information for calculating cable sizes in relation to circuit disconnection times.

Reply to
ac1951

You need to know the PSC to enable you to select appropriate MCBs, ie capable of breaking the fault current. You need to know the external loop impedance to ensure the total fault loop impedance complies with the (corrected for temperature) tabulated value ensuring disconnection times. Do not forget that the resistance you measure in a cable is when the conductors are cold, Under full load working conditions they could be at 70 degrees C and thus higher resistance.

Unfortunately the Electricity Suppier will quote maximum values as laid down in the supply spec and this is of little use in the real world. For instance in the majority of the country a PSC of 16000 amps will be quoted for a 100A domestic supply, even though in practice most domestic supplies will rarely deliver more than a couple of thousand amps due to conductor resistance in the distribution network. A maximum Ze of 0.8 ohms (TN-S) or 0.35 ohms (TN-C-S) will be quoted. If you consider a 230 volt nominal supply the open circuit voltage will be taken as 240 volts. To calculate PFC at 0.35 ohms divide 240 by 0.35 and you will find the answer (686 Amps) a very long way short of the 16000 quoted, obviously even less with TN-S systems. To get significantly high PFCs you need to live virtually next door to the transformer. When you ring the call centre you usually get a droid who tries to fob you off. Insist on speaking to the supervisor and remind him/her that the Supplier has a duty under the ESQCR to furnish you with the information. Your answer will come from the "supplier" who you pay your bill to, not the RDO

Reply to
cynic

If you look at the taking electricity outside wiki article it gives and example of how they can be used to good effect - working out what fault currents can be anticipated at the end of a cable run and hence the time take to open the protective device.

Although they do insulated and metalclad - the latter not usually used on TT installs.

More MCB operation times rather than ratings, and "fault withstand" (adiabatic equation) times for circuit protective conductors.

Reply to
John Rumm

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