# Application of BS7671 Table 54.7

Can anyone offer experience on the application of Table 54.7 for Main Earth size on non PME supplies for supplies that have parallel incoming cables.

For example, 2 x 185mm2 PCV/SWA installed in separate ducts.

Thanks

• if each of your line conductors is 2x185mm² then plug in 370mm² which then gives you 185mm²

Thanks Andy, this is the method that I would adopt if I needed to use this method. However, there is a caveat that I would employ in the application.

1.  I would size the incoming cable on the first 10m. Reason for this as after this the incoming cable size can balloon due to voltdrop... 10m as this is the typical length that most DNO would allow the cable to be run without protection.

Absolutely agree, adiabatic equation is the best solution, I am really just quizzing the application of table 54.7 and whether it could be used. Should be is a different question.

• I may be completely naïve here, but if you want to be lazy, 543.1.1 allows you to use table 54.7. In your example, given that the minimum cross-sectional area of the protective conductor = S/2, one of the cables needs to be accompanied by a protective conductor of the same size as the line conductor(s). Alternatively, each one could be accompanied by a protective conductor of S/2. Note that the separate protective conductor will not protect the cables between the supply and the load or downstream DB.

That said, 543.1.1 goes on to say that if the size of the line conductor has been chosen by considering the PSSC, 543.1.3 must be applied rather than table 54.7.

That's how to make an installation compliant, but perhaps it is better to start with the proper engineering principles, which are a bit above my pay grade.

• Chris, it not really about being lazy ... its about understanding the applicable application of Table 54.7 and whether it can be used (and application of) for Main Earth size on non PME supplies for supplies that have parallel incoming cables.

I do appreciate you comment with respect to the fact that calculation must be performed where line conductor has been chosen by considering the PSSC.
Hence on these occasions Table 54.7 must not be used.

• I may be missing the point of the question. When you say non-PME, is that TN-S or TT?

543.1.3 may always be applied. As I understand it, 543.1.4 (and table 54.7) may be applied except as already mentioned. If you have a choice, it's up to you.

• Many answers on this thread take the problem sideways and make the question more complicated than it is. I am really trying to keep it a simple question ....

The Question is - "Can Table 54.7 for Main Earth size on non PME supplies for supplies that have parallel incoming cables?"

The other details provided were hypothetical at best, to give readers a basic understanding of what I was thinking.

If you consider that TN-S or TT gives different answers to the question perhaps you can comment on both?

• Well clearly with TT, the core can be much smaller, as the fault current path includes the current limiting resistance of the ground, several ohms is likely, so a cable good for 100A might  do - 10mm perhaps ;-)    also you'd need an earth fault relay, as there is no chance of blowing a 1000A fuse, an LE fault in such a case without the Earth fault detection just leads to high bills, dry hot earth around the electrodes, and nasty step voltages as nothing disconnects.

With TN-s similarly, the impedance of the earth path will limit the current, but depending on the physical construction how much metal is involved, it may be more or less than the PME case.

Of course you have not said in  your PME supply where you expect the N-E bond to be - a lot of large sites with a private transformer actually do not do this at the transformer, but at first breaker or fuse panel - this also simplifies things when multiple transformers are being paralleled.

Mike

• Mike, whilst I understand the difference in fault currents and the how that may affect the incoming supply cable, but with respect to the question where I am specifically trying to focus on the understanding of Table 54.7...

Can Table 54.7 for Main Earth size on non PME supplies for supplies that have parallel incoming cables be applied for TT systems?

If the system is a TT is the method of application different from a TN-S system?

• This question is with respect to understanding the application of:

• Table 54.7
• Parallel incoming cables.
• Non PME supplies (so all supplies other than TN-C-S)

This question does is not with respect to: