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C40A Tp Mcb, High Zs


mooseoff

Question

Hi, 

 

I have a C40A three phase circuit (final) with a measured Zs of 0.8 Ohms (0.55 max). Obviously this is too high as per table 41.3.  The supply is a TN-C-S with a measured Ze of 0.71. Obviously the R1+R2 should be around 0.09, and low and behold was measured as such. There is a 300mA RCD in place adjacent to the 3 phase supply cutout (300mA due to sensitive equipment and requirement for no nuisance tripping as its a remote site) 

 

There is additional protection afforded via a supplementary equipotential bonding conductor to the equipment. 

 

My query is does this pass to issue a cert? I'm not the one signing anything of btw, just want to expand my knowledge. 

 

Cheers Chaps

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5 answers to this question

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What does this final circuit supply and where?? are the cables buried in walls and how deep etc...

 

You need something to provide ADS within the required time in the event of an earth fault,, and i assume that the RCD will sort this, but what if some of the stuff supplied requires a 30mA RCD for shock protection purposes.

 

Also, although there is nothing in the regs about disconnection times for L/N faults, these require disconnection within 5 seconds too, as, the formulas in the regs for the heating of cables in the event of a fault are all based around a max disconnection time of 5 seconds. You can calculate what the cables will withstand for other disconnection times, and there is a BS that deals with this, but i have no idae which one and certainly have not got one.

 

Some of the others will no doubt give you a better answer, but they will need more information first!!

 

john..

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not wanting to state the obvious, but how about fixing the problem of Ze being twice its max limit...

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Good answer!!!!!

 

john...

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not wanting to state the obvious, but how about fixing the problem of Ze being twice its max limit...

 precisely, probably just a loose terminal, not exactly hard to check. Are all 3 phases the same?

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^what binky says... :)

Also are you sure that it's not 0.08 ohms?

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