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Checking adiabatic calculation

Alan B

HI 

Just want to check a calculation and eicr coding with you as I very rarely use the adiabatic equation.


I have a domestic incoming supply where the feed from the meter box to main DB is protected by a B63 MCB. 16mm tails and and 6mm earth run through ground floor ceiling. about a 6m run
TNCS supply with 100A fuse, all main bonding (10mm)  goes back to MET located in meter box, therefore assuming majority of diverted neutral current would not go through 6mm cable to main DB.

Originally installed as TT (rod still connected), someone converted it at some point. Type 1/2 SPD installed in meter box.

Grade 2 listed building with polished oak floors, would like to avoid cutting and lifting them if possible.

Incoming supply Ze 0.21, measured PFC 1.46 (measured just after meter)

S = √(I2t)/ k

S = √1460*1460*0.1/115  =461/115  =4.01mm2. Therefore 6mm ok.

Admittedly I don't know how the cable is installed and if it's 50mm from floor and ceiling.

Overall based on what I can see I think it's acceptable for an EICR (not how I would install it)

Thoughts?

  • 0

    Calc is OK, 6mm will be fine  for 1400 A fault currents that take  100msec to break, and most breakers will be faster. It won't protect you as you note, against diverted neutrla currents or other odd things that inject external fault currents into the earth, but then nothing will really as no breakers open the CPC ;-)
    But, another way of looking at this for diverted current, does the insulation look cooked, does the copper look blue ? if not then large currents are not occurring.

    I'd agree it is fine for continued use. Nowadays it would probably be 25mm tails and 16mm earthing, and if the supply ever needs upgrading it can be done then - for now its not a real issue. If you are worried it may have hidden high Z joints or something repeat the PSSC test at the other end. It should differ by very little.

    Mike.

  • 0 in reply to mapj1

    Mike

    Thanks for the confirmation. No evidence of heat issues. Zs and PFC at other end of cable as expected. There is a joint made with connector strip that I will replace with a crimp.

  • 0 in reply to mapj1
    mapj1 said:
    Calc is OK,

    Calc is not strictly conformant with BS 7671, for circuit-breakers with disconnection time < 0.1 s. The let-through energy quoted by the manufacturer, or the standard, is to be used.

    I think in this case, the conformant calculations would yield similar results, but it's not always the case.

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