Student doesn't understand disconnect times

[mhar]

Why do circuits over 32A have a 5s disconnect time yet those 32A or less have a .4s disconnect time? I have read the relevant regs but do not understand why and cannot find the answer in my books

 

[Deleted member 26818]

It's mainly to provide discrimination between protective devices, partly to do with cost and very slightly to do with safety.
Most circuits above 32A are used for distribution to final circuits. As such it is preferable for a fault occuring on a final circuit, not to cause the protective device for the distribution circuit to operate.
Some circuits above 32A are final circuits, but as they are generally used to supply fixed equipment, the likelyhood of damage occurring to flexible leads is slight, and the cost of providing a 0.4s disconnection time prohibitive. Also many of these circuits are required to be additionally protected by a 30mA RCD, so the disconnection time becomes rather moot.
It used to be in the 16th, that the 0.4s disconnection time only applied to circuits supplying socket-outlets or other circuits supplying hand held or portable equipment.
Obviously a 0.4s disconnection time is safer than a 5s disconnection time. Lighting circuits were allowed a 5s disconnection time.
The 0.4s disconnection time was extended to cover lighting circuits in the 17th, to provide greater safety, probably because of the increase in the use of metal light switches, and the increased use of socket-outlets for connecting standard lamps and table lamps etc.

 

[mhar]

exactly the type of reply I was hoping for. Many thanks

 

[telectrix]

still see test sheets where the electrician has entered 5 secs. for lighting.

 

[johnboy6083]

as spin said! if you tried to get a Zs on a large feeder low enough to allow a shorter disconnection time, thecable size wouldhave to be largher, and your talking lots of money!

 

[mhar]

Yes johnboy. Understood that. Point was that the regulations aren't concerned with cost, they are there to govern safety. I knew there must be a practical reason.

 

[johnboy6083]

very true, but they are compiled by lots of different parties, all with different vested interests.

 

[telectrix]

vested, also include suited, tied, shirted, kept in booze, mistress in £1m apartment, bi-annual thailand trips........