Prospective fault at the end of circuits

[John-]

Does anyone perform a prospective short circuit and, where no RCD / RCCBO, a prospective Fault Current test on the end of circuits on inspection and tests?

I get we do ZS, but how do we know that the CPC or Line conductors are man enough for fault currents of X duration?

S2K2 => I2 T?


Cheers

John

 

[pc1966]

If the circuits are "standard" and it is fed from the usual style of MCB then they would meet the adiabatic limit.

Table B7 in the OSG has the limits based on the generic MCB standard and differing fault currents in terms of CPC size. Though even the 3kA one is more than you would typically see at the end of a final circuit.

Section 8.5.1 starting on page 98 of the IET's Electrical Installation Design Guide (4th edition) has more detail and includes example MCBs from MK & Hager that have lower let-through than the limits of the generic specification.

TL;DR using reputable T&E sized for the MCB is sufficient is end Zs met.

 

[John-]

Sent in error

 

[i=p/u]

the adibiatic is all worked out in standard circuits. But work it out and save on copper misuse, win more work as cheaper.

 

[Julie.]

The max fault level at the end of the cable is largely irrelevant, the Zs/minimum fault level of course is vital to ensure the ocpd operates in time.

If we put a nice long cable run, there is no guarantee that when the user drives a nail between line and cpc they will bother walking that full distance to the far end to do it.

They will probably do it close to the consumer unit, therefore if you do check the adiabatic on the cables, you need to use the fault level from the start of the run, not the far end.

So say I have a Re of 0.08 ohm ( so around 2.7kA on min voltage - 3kA max), we need to make sure the cables survive for 3kA perspective through their ocpd, rather than just the 45A at the end of a long lighting circuit having a 5 ohm Zs, which is within the 6A rcbo/mcb limit.

The standard arrangements assume a maximum let-through of the main fuse limiting the damage, so providing you fit mcbs capable of handling that (usually 6kA), then the standard circuits in the standard, and osg can withstand the let through of these mcbs.

This might not be the case in industrial settings as the fault level may be higher, with substantially bigger fuses, and larger let through of the industrial mcbs.

Hence in these cases you do need to check the adiabatic for seemingly "standard circuits"

 

[old man sparky]

Slightly off topic but it amazes me how often within commercial applications design load currents have been factored around the ratings of say XLPE cables and then they wonder why the protective device mcb or mccb has tripped prematurely. Although these XLPEs can operate under higher temps, hence load currents. They fail to take into account the thermal affects of those cables and how they can affect time current characteristics of the protective device if its taken outside specified ambient operating temps. Another one is those type Ds replacing a C type without the circuit Zs limits being taken into consideration. As said often overlooked ?

 

[Lister1987]

BS7909 does both PFCs and EFLI at the furthest point from the distro gear for safety reasons (ensuring sufficient impedance and fault currents to operate the protective device, and helps confirm volt drop too.