busbars and upstream overcurrent protection

[James sparks]

Hey all

Want to gain some opinions on busbars and upstream protection.

situation:

3phase busbar chamber, fed from a switch fuse on a rising busbar system. I'm unable to turn off the switch fuse to see what size fuses are installed as it would kill everything on the busbar chamber, but there's a few DB's and what not on the chamber so I'd guess at min they're 100A in the switch fuse.

A small single phase DB is tapped off the busbar chamber in 6/10mm singles which go straight in to the main switch of the DB which is just a double pole disconnector. The circuits are then fed from the small DB with mcb's 6,10A and a few spares etc.

I'm thinking this is wrong. The fuses in the switch fuse are possibly 100A min, the 6/10mm cable is only rated 40/50A and if something went wrong there's no fuse inbetween, protecting the 6/10mm, to blow. The main switch in the small DB is only a disconnecter and offers no overload protection.

Also, to isolate the small DB you have to shut off the whole busbar chamber. So there is no local isolation.

I thought it was ok to tap off the chamber in a cable that is less than the rating of the fuse before the chamber, if you then had eg. a switch fuse upstream (Eg. after the chamber and before small DB) that would have the correct size fuses in for the smaller cable going off therefore protecting them/limiting what can be drawn through them.

It's been a while since I've had much to do with bubars, it's all MCCB panel for me of late. So I'd just like to get some opinions as to wether i'm thinking along the correct lines or not.


This is also very close to the origin and so the highest PFC I got at the small DB was 10kA. All the mcb's are 6kA.

Any help is much appreciated
Thanks

 

[johnboy6083]

isolation is provided by the main switch of the DB. backfusing is allowed to a max of 3m, buit there must be upstream overcurrent protection of the smalled CSA cable. Change the DP isolator in the small DB for a DP MCB, unless you can guarantee that there is no chance of overload. Short circuit protection is provided by the upstream Switchfuse, so your 10Ka fault current will be dealt with by that. Its called cascading.

 

[James sparks]

Hi Jon

Thanks for your reply. Just need to clarify a couple of points

DP MCB - cool, that's what I thought as then there is overcurrent protection for the 6/10mm (what could you do to guarentee no chance of overload?)

backfusing - can you clarify what you mean please? Is that where overcurrent protection is upstream?

local isolation provided by the main siwtch of the DB. - But if you wanted to completely isolate the board for whatever reason you have to shut the entire busbar chamber and everything else coming off it, down.

10kA PFC(PEFC/PSCC) - this reading was taken at the small DB not at the origin. Could you tell me a little more about cascading please. The switch fuse is before the busbar chamber:
origin:meter:glasgow:rising busbar:switch fuse:busbar chamber:small DB

thanks for your help!

 

[Darkwood]

The tails off the busbar must not be more than 3m long, if this is satified then you can fit a DP mcb to protect the tails in downstream of the tails instead of upstream, then it not possible to expand the circuits and accidently exceed the tails rating.
see 433.2 and 433.3
These regs allow small cables to be attached to large busbar systems, let say a fused isolator fitted to a 400amp busbar does not have to have the impossible task of fitting 400amp rated tails in to a 100amp isolator you can derate the tails to suit as long as they dont exceed 3m, they can not overload as the fuse in the isolator downstream of the busbar would prevent this.

 

[James sparks]

Thanks Darkwood, that's very helpful. Just checked out those reg no's. 433.2.2 and the 3m rule - exactly like your scenario above describes!

any ideas on the PFC? if i'm reading 10kA at the DB, i thought the main switch and mcb's had to be rated to this?


thanks!!

 

[Darkwood]

Your busbar will most likely have bs88 fuses with an 80ka rating this affords you the protection you require, any fuses downstream dont have to meet this requirement as long as at least one fuse in the line UPSTREAM! does, so under a fault if it destroys the mcb and welds it closed circuit the bs88 fuse wont be destroyed but will function correctly breaking the supply.

 

[Darkwood]

Thanks Darkwood, that's very helpful. Just checked out those reg no's. 433.2.2 and the 3m rule - exactly like your scenario above describes!

any ideas on the PFC? if i'm reading 10kA at the DB, i thought the main switch and mcb's had to be rated to this?



thanks!!

in your senerio it is open for overating of tails by additional circuits been added so fusing the tails down will be best but in this case can be done downstream of the busbar by swapping the main switch with a double pole mcb/rcbo as one example. If this isnt within the scope of price of job then identify the dist board and affix max load allowable of boards use ( Total load of this board must not exceed 40amps), this gives insight to any who follow.

 

[James sparks]

fantastic!

I think i've been getting confused with upstream and down stream DOH!!. Down stream is away from the origin and upstream towards the origin, is that correct?

just read an article on cascading (PFC kA ratings) and it's as you describe. They're refer to it as 'back-up protection'. From what I can make out it's good to match or better the PFC value where possible but where you can't (i've only seen mcb's go as high as 15kA), you can rely on back-up protection somewhere further upstream. So in this instance, like you say there will be suitable rated cartridge fuses in the switch fuse on the rising busbar or failing that in the glasgow main switch.
I think personally, if I had installed the small DB I would have got one with 10kA breakers to match the PFC as it's possible (they are readilly available).

Thanks for all of your help! I am happy with where I stand now

 

[Darkwood]

fantastic!

I think i've been getting confused with upstream and down stream DOH!!. Down stream is away from the origin and upstream towards the origin, is that correct?

Yes just remember upstream / up the mountain to the source and down the mountain to the sea (end of journey) ... yeah i know crap analogy but it helped me in early days.

 

[James sparks]

That works for me! I was getting confused because when I draw a schematic the origin is at the bottom of the page not the top like the mountain.

thanks