0.4s and 5s

Hi guys could u pls explain what the purpose of the 0.4 and 5 second disconnection times are for. I understand that the zs obviously has to be low enough for the protective device to trip in time but why does it only apply to circuits upto 32 amp? And when does 5 seconds disconnection apply? In the BRB table 41.4 has max zs for bs88, 1361,3036 and 1362 only. Is it only for distribution circuits or does 5s apply to all circuits over 32 amps? Thankyou. :Salute

5 sec is for distribution circuits of & above 32amp on a TN system.

Ok and what about non distributing circuits over 32A?

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And if this is 5s, why? Surely this is more dangerous?

5 sec is for distribution circuits of & above 32amp on a TN system.

Click to expand...

All distribution circuits and final circuits greater than 32A.

but not showers

Yes, and showers if protected by >32a device, ie: 40A MCB .

Not forgetting the 0.2s limit on TT; unless certain conditions apply

gordy71 has a point why such a long time 5 secs for distribution circuits

Thankyou peetee this is what i was getting at. Surely a higher current carrying circuit is potentially more dangerous under fault conditions, so why give it 5 seconds to bad day explode you? And surely cables with larger surface area of copper will return betterohms

I would expect it is down to the time/current characteristics of some OCPDs ( usually NOT MCBs or RCBos) ; which, in connection with the current-carrying capacities of the cables, means that the cable will be deemed capable of dealing with a fault of a magnitude such as to need 5 seconds to remove supply from the affected circuit. It allows allows for discrimination; whereby a final circuit which is supplied by the relevant distribution circuit, can isolate a fault before the upstream OCPD "trips".

HTH

KME

have a look at 16th edition,

lots of instances of 5s disconnection with a bit of a reason why. (I think....)

Thankyou peetee this is what i was getting at. Surely a higher current carrying circuit is potentially more dangerous under fault conditions, so why give it 5 seconds to bad day explode you? And surely cables with larger surface area of copper will return betterohms

Click to expand...

As stepse implies.. and IMHO the 'WHY' bit of 0.4 sec -vs- 5 sec Disconnection times comes back to some of the historic reasons in earlier regs..

e.g. If you look back at the yellow cover On-site guide 16th edition page 19 it states that a 0.4 disconnection time is required for all final circuits supplying:-

i) Portable equipment intended to me moved by hand while in use.

ii) hand held metal cased equipment requiring an earth supplied though a socket.

iii) Fixed equipment outside the equipotent zone.

iv) fixed equipment in bathrooms.

As we then move toward the brown cover 16th edition O-S-G..

page 19 is the same except it excludes the item iv) bathroom equipment.

(which is probably covered by RCD anyway!)

Now as we get to our current red book 17th edition (page 21) it is just ALL final circuits up to 32A..

So IMHO a large proportion of this pivots around the realistic likely risk of electric shock and minimising it down to a short duration, preventing the build up of dangerous touch voltages, to avoid a shock becoming a fatal shock!

So If you did a general risk assessment of the various circuits and those circuits more likely to give an electric shock Final circuits over 32A and Distribution circuits are quite lower down the risk list..

i.e.

Mr