5 second disconnection time

[La Poste]

Just a quickie to refresh my memory.

TN system disconnection times.

Final circuits not exceeding 32 Amps.
230V 0.4 seconds. (table 41.1 page 46 BRB)

Final circuits exceeding 32 Amps IE fixed equipment and distribution circuits.
230V 5 seconds.

RCD's are used for socket outlets not exceeding 20 Amps and mobile equipment with a current rating
not exceeding 32 Amps for use outdoors.

Just clarifying.
Is this correct, are there any more, any exceptions?
Thanks.

 

[Deleted member 26818]

For socket-outlets intended for general use by ordinary persons.

 

[scotsparky]

Any final circuit in a special location as it has to have RCD protection. so 0.4s at 150ma

Street lighting as every light has individual fuse

 

[ackbarthestar]

RCD's are used for socket outlets not exceeding 20 Amps and mobile equipment with a current rating
not exceeding 32 Amps for use outdoors.

Exceptions to socket outlets being:
1/ Under the supervision of a trained/skilled person
2/ Specific item of equipment that would benefit from not being RCD protected

 

[La Poste]

What I can't understand is that in the back of GN3 page 108-110 there are tables with a selection of fuses. BS3036, BS88, BS1361.

These tables are split into two parts, one being 0.4 second disconnection time and the other being 5 seconds disconnection time.

If we look at table 41.4 page 108 it lists BS 3036 with 5 second disconnection times and starts at 20 Amp fuses, where would a 20 Amp fuse be used for a circuit with a disconnection time of 5 seconds.
These tables seem to be in contravention to BS 7671.
I don't understand.

Also there are tables for BS 88 fuses with 5 second disconnection time starting at 20 Amps and BS 1361 fuses with 5 second disconnection times starting at 20 Amps.

So according to these tables there are instances where you would put a 20 Amp fuse in a circuit with a disconnection time of 5 seconds.

I'm confused, help. :dozey:

 

[scotsparky]

If you relate the disconnection time to the time current curves in appendix 3 that will show that on a 5 sec disconnection time you are allowed highter Zs values. so the 5 sec Zs is higher then 0.4 values

lower the value =higher PSC = Faster disconnection


In a street lighting circuit you can use HRC fuses for a 5 sec disconnection time.

 

[La Poste]

So would you use a 20 Amp fuse for a 5 second disconnection time on a street lamp?

 

[ackbarthestar]

There are still many installation distribution boards (industrial) that have mixed disconnection times and for smaller fused circuits such as lathes and other fixed industrial machinery combined with socket outlets.

 

[scotsparky]

i would use it as the street lamp supply yes. remember that street lights have individual fuses therefore the supply could be classed as sub main distribution.