minor works cert help

[sinaloan]

hi there i am filling in a minor works cert and in part 2 it asks for 'overcurrent protective device for the modified circuit' and 'residual current device'. i have a split load board with one rcd on it. the work has been carried out on a ring circuit on the rcd side so i assume i put in the rcd box i fill in the rcds details? in the 'overcurrent protective device for the modified circuit' box do i put in the mains switch bs number 60947-3 or do i put in the main fuse bs number 1361?

 

[sinaloan]

so the allowed pfc is up to 16.5ka?

 

[Goody]

...but iv never understood pfc and what its about and what the reading should be or mean. my college were rubbish and i learnt more over the three years researching myself.

The prospective short circuit current (PSCC) is the highest electric current which can exist in a particular electrical system under short circuit conditions. This value dictates the type (capacity) of breakers used in the circuit.

When a large electric current is interrupted an arc may form and if the breaking capacity of a fuse or circuit breaker is exceeded then it may not be able to extinguish this arc meaning that the current will continue, possibly resulting in a fire or explosion.

 

[topquark]

so the allowed pfc is up to 16.5ka?

It will depend on the rating of the main protective device in use (which is normally the DNO fuse).

 

[sinaloan]

so if the main fuse is 60a then my pfc could be?

 

[telectrix]

no. the pfc should not exceed the breaking capacity of the fuse, not it's rating. the DNO fuse capacity is usually 16.5kA, but it deoends on the type of fuse.

 

[sinaloan]

cool ok sorry i got ya now thankyou all for the help i really appreciate it

 

[Guest123]

Have a read of this snippet BS 60439, Annexe Za.....


"Part 3 of the Standard, BS EN 60439-3, deals with the particular
requirements for low voltage switchgear and control gear
assemblies intended to be installed in places where unskilled
persons have access to their use. This covers the supplementary
requirements for enclosure distribution boards suitable for indoor
use containing protective devices intended for use either in
domestic applications or other locations where unskilled persons
have access. Control and/or signalling devices may also be
included.
These distribution assemblies are for use on AC supplies, with a
nominal voltage to earth not exceeding 300V. The outgoing
circuits include short circuit protective devices, each having a
rated current not exceeding 125A with a total incoming load
current not exceeding 250A.
In the UK, such equipment is referred to as a ‘Consumer Unit’
and as such is covered by this Standard. However, additional
requirements from annex ZA of the standard call for the
assembly to have an additional test; this is known as the
‘conditional withstand test’. The condition is that the consumer
unit must withstand a 16kA short circuit fault when protected by
a 100A HRC fuse to BS 1361 type II. Further requirements are
addressed by this annex:
• Means of isolation to be via a manual double pole switch
disconnector
• Rated current of consumer unit is determined by current
rating of incoming device.
• Outgoing protective devices can be MCB’s, Fuses and/or
RCBO’s."

 

[topquark]

Maximum is normally dictated by the DNO's fuse max rating. I believe that the maximum recommended for them is around 16.5 KA Most main switches 60947-3 are rated up to 6 KA (and it should be marked on each BS standard compliant device).

Ooops, that should be minimum!! I'd best fix that

 

[sinaloan]

so if not diversity factors are applicable to consumer units then how do i work out my maximum demand?

 

[sinaloan]

so if my pfc was over 6ka topquark then id have to upgrade them as this would exceed the breakers ssc?

 

[topquark]

so if not diversity factors are applicable to consumer units then how do i work out my maximum demand?

Diversity may be applied to all circuits that are supplied by that consumer unit.

 

[Guest123]

so if not diversity factors are applicable to consumer units then how do i work out my maximum demand?

Dont worry about that bit, circuits eminating from the consumer unit may have diversity applied. I'll edit it out to remove any confusion.

 

[topquark]

so if my pfc was over 6ka topquark then id have to upgrade them as this would exceed the breakers ssc?

No, as you are protected by the DNO's fuse breaking capacity, assuming that is large enough. As stated they must have a minimum of 16 KA for a 230V system (I think it's 33 KA for 3 phase).

You should not rely soley on the DNO fuse for overcurrent though, if that's what you are thinking, they don't very often fail that quickly at rated current capacity (an 80A fuse may supply current at 100A for some considerable time) and you wouldn't want to break the seal to replace it after testing would you now

 

[sinaloan]

ok cool so the best way to work out my maximum demand is to use the diversity factors in appendix 1 of osg. does everyone do it like this or is there an easier way to work it out, someone told me to just put down the rated current of the main fuse but thats wrong right?

 

[Des 56]

I think this thread is getting to be like that pantomime song

"There's a hole in my bucket dear Lisa, dear Lisa"

and finishes

"There's a hole in my bucket dear Lisa, dear Lisa a hole"