MCB Capacities - Will my MCB's melt or Explode ?

[Guffer]

Testing a sheltered scheme with some old MEM boards, black mcb's, white button, little red button you know the ones. Have the folowing Icn values 5A - M1 anything above M1.5.

Lovely SubStation amongst the bird tables out back so PSCC is up there at 4.summat kA

Does this mean on a short circuit these have the potential to weld shut, if so i assume this will be a C1 on the cert and i'll have to change the dam things or lock the cupboard and put LiM ?

Any Thoughts

 

[Lighthouse Phil]

I've seen Crabtree breakers melt and re-make on a short circuit. Your DNO fuse will break before the breakers are damaged.
Not in the UK so not sure on your local regs re : certs

 

[robsparky1975]

I would say if the measured value is 4ka then id do the same,,and note the relevent code and recommendations on the test cert,why would you lock the cupboard and put lim on the sheet,you have seen it and if you deem it insufficient to provide adequate circuit protection for continued use then id put it down 100%.your the professional and give out the best advice that you can.If someone can not afford to replace then you have it down on paper and thats where your part ends.

 

[Guffer]

Thanks for the reply Rob, I agree and i'm pretty sure we will change the boards the finance is not an issue. The lock and LiM were just a joke by the way

 

[robsparky1975]

Thanks for the reply Rob, I agree and i'm pretty sure we will change the boards the finance is not an issue. The lock and LiM were just a joke by the way

Phew, so glad it was a joke lol

 

[spark 68]

You may be OK if the switchgear is single phase and up to a 100A and a type tested ---'y (CU), to BS EN 60439-3 or BS5486 See 530.3.4 and p57 GN3.(p 54 of the old GN3)

BS EN 60439-3 Annexe Za or BS5486 states that the Icn of the sub parts of the CU may be taken as 16kA provided the OCPD's don't exceed 50A and if it is supplied via a Type II fuse (DNO fuse etc.) to BS1361:1971 or BS 88-3 rated at no more than 100A.

Edit: the CU is then classed as a complete ---'y rated @ 16kA

Does anyone know which page annexe ZA is on ?, I could have sworn it was in the BRB or the BGB but I cannot find it now.

 

[Guffer]

Thanks for that Spark 68 i will go away and do some more reading. Theres a footnote in one of the Nic guides about classing the CU @ 16kA when single phase and 1361 etc. However in this case these are small dist boards south side of two or three Main 3 phase DB's and intake fuses at 250A so not quite sure how to interpret that.

Reason im shying away from simple upgrade, these DB's have multiple Micc's from all directions not touched for 40+ years and past experience says leave well alone. Suppose im just getting lazy in old age.

Thanks again

 

[spark 68]

Does the 250 A fuse/s just supply the larger DB's ?, and what OCPD feeds these smaller DB's ?

For instance if the 250A fuses are the mains incoming to the larger DB's you may find that the smaller DB's are fed from a lower MCB/Fuse etc.(in the larger DB), and so comply anyway (within the 16kA CU rule).

 

[Guffer]

Yes the 250A is main switch fuse at intake feeding 1st DB then i think BS88 100A type 6 @ 80kA then next DB BS88 63A type 2 @ 80 kA then the DB in question. Like i said i'll have a read to get it straight in my head so i can justify and leave well alone.

Much appreciated and thanks again that may have just saved a great deal of hassle.

P.S. Biggest fear Communal TV Amp off DB, have you seen 40 old dears who cant watch deal or no deal and your the culprit !

 

[Risteard]

It may come under the 16kA conditional rating which is a UK derogation.