16mm T&E cable on distrubution circuits - The 6mm CPCs are a limiting factor no?

24EMan · Nov 18, 2023

Hello all,

I wonder if I can get some opinion on my deliberations on an old TPN installation with numerous 1P sub-boards wired up with 16mm T&E cables from a TPN distribution board. It's getting well beyond it's time for the wiring but the client (commercial) needs to eek things out for another couple of years before selling where there's no doubt then that the property will be flattened to build new fancy apartments. So a total rewire is not on the table.

Main board is the old metal MEM TPN DB with 3871 type 2 & 3 devices and the sub boards are MEM Metal consumer units with 3871 plugglable devices where the old 3036 fuses were first installed.
I certainly wasn't around for 13th or 14th Edition or whatever this was designed to but making some signficant changes in upgrading switchgear in the sub-boards that is absolutely necessary might mean current calculations won't permit continued use of the 16mm T&E without reducing the power on the OPD protecting these distributions circuits. The main TPN board won't be changed for new so the 3871 OPDs will have to remain protecting the disbribution circuits.

The distribution circuits from the main TPN board are currently on 63A 3871 Type3 breakers. The CPC is the 6mm conductor in the T&E cables.

Using the adiabiatic for the CPC size on the distribution circuits I get the following:

Earth fault loop current I = (230 x 1.1) / 0.35 where the 0.35 is max Zs for the 63A BS3871 Type3 OPD
k = 115
Disconnection time is 5s

so the adibiatic spits out the result as 14.055 so the minimum size CPC as 16mm. Working things backwards the maximum fault current for a 6mm CPC should be 308A . So if I upgrade the Sub-boards I may have to reduce the size of the OPD on the distribution circuits where the minimum Zs for the OPD at the main board would have to be

Zs = (230 x 1.1) / 308 = 0.822

The BS3871 Type3 OPDs on the TPN board for the distribution circuits would then have to be no more than 25A rating, or if I can use Type2 this can be 32A. The demand on some of the sub boards may be low enough to get away with this, but I was wondering if you more learned gentlemen might indicate what part of my thinking is wrong where current reg's would allow continued use of the higher value OPD's on these disbribution circuits using the existing BS3871 main board.

westward10 · Nov 18, 2023

I haven't done any calculations but maximum Zs of 0.35 is based on 0.4s not 5s.
T&E cables have been engineered to have a reduced cpc and the chances of it not being adequate are pretty slim.

24EMan · Nov 18, 2023

westward10 said:
I haven't done any calculations but maximum Zs of 0.35 is based on 0.4s not 5s.
T&E cables have been engineered to have a reduced cpc and the chances of it not being adequate are pretty slim.

So even though the distribution circuits have a maximum disconnection time of 5 seconds, because an MCB to BS60898 and most likely BS3871 will magnetically trip in less than 0.4 seconds anyway we use that disconnection figure in the adiabatic calculation for final and distribution circuits ? Using 0.4 we get a result of 4mm in the above calculation.

westward10 · Nov 18, 2023

Essentially yes because you are using 0.35 (0.4s) as the maximum Zs. Establish the maximum Zs for 5s you will probably get the same or a similar result.

pc1966 · Nov 18, 2023

I don't have any worst-case figures for BS3871 Type 3 MCB but this is the let-through I2t for a modern Hager equivalent for C-curve MCB:

[ElectriciansForums.net] 16mm T&E cable on distrubution circuits - The 6mm CPCs are a limiting factor no?

For most faults it is less than 300k A2s of let-through, giving around 4.8mm as your minimum CPC size (from k=115 off Table 54.3) so to all intents and purposes that is OK.

Most likely faults will be final circuit, so the sub-boards will further limit the I2t seen by the sub-main. Also these days I doubt anyone can afford to run the T&E at 70C starting temperature either, but that is not an engineering design aspect!

Pretty Mouth · Nov 19, 2023

BS3871 type 3's have a similar shaped operating curve to BS60898 type C's. Any fault that will meet the disconnection time of 5s (for 63A breaker, this would be a fault of at least 630A) will cause the device to operate in 0.1 seconds. So for your calculation, use t=0.1.