3 phase dis board

[eagleeye]

I've been doing a big job at a wharehouse and had to add 5 additional circuits to the dis board. The board is now full but i need to add more circuits for some other work that needs doing. Its just a couple of hand dryers and some low wattage outside lighting. Whts the best way to go about supplying the additional circuits ? Do I fit a new bigger 3 phase dis board or just fit a single phase cu off of one of the phases ?? And would it be ok to split one of the phase tails before it goes into the 3 phase switch feeding the dis board ??

 

[telectrix]

do NOT split a phase tail on incoming side of board. take your sub-main off a MCB in the board.

 

[Smudge]

i would feed a sub board from an MCB, and migrate some circuits onto it, ie/ lighting. It may take a little time to re-configure, but definately better than changing the complete board. Don't split incoming tails.

 

[telectrix]

and check the loading on each phase. try and balance the loads as evenly as possible. this may require some jiggling about of the various loads. if you are lucky, you may find 1 phase is under loaded comared to the other 2, so add new loads to that one.

 

[johnboy6083]

I would change one of the ways in the 3ph DB to a fuse carrier, and feed a new 1ph db from that. The reason for this, is that if you have a short circuit on one of your circuits in the new DB, then if you have 2x MCB';s in series, then it will probably take both out, so you lose all of your circuits on the 1ph DB. Fuses will discriminate with MCBs on overload and short circuit, so you should be ok. Go with the 2:1 rule and you should be ok.

 

[telectrix]

I would change one of the ways in the 3ph DB to a fuse carrier, and feed a new 1ph db from that. The reason for this, is that if you have a short circuit on one of your circuits in the new DB, then if you have 2x MCB';s in series, then it will probably take both out, so you lose all of your circuits on the 1ph DB. Fuses will discriminate with MCBs on overload and short circuit, so you should be ok. Go with the 2:1 rule and you should be ok.

if he's putting 3 or 4 circuits in his sub-main, that will achieve discrimination, subject to MCB ratings. if in doubt, the upstream \mcb could be a C type, at around 1.5 x the rating of the highest sub-main MCB.

 

[johnboy6083]

i suppose it depends on the fault currents. we know that a 6A B type 60898, will trip at 0.1 secs if a fault current of 30A flows. If we use the C typeof 1.5 x the downstream to feed this, then we will have a 10a,C type, which will trip at 0.1secs at a fault current of 100A.

If the short occurs close to the origin of the circuit, and the Ze is relativley low, then we could still end up with both devices going.

 

[Darkwood]

Would need to know more info about d.b. what make is it as some boards do inline busbar link unit that can feed direct to a board fitted next to it, discrimination then is the same as existing boards front end supply fuses. Its normally not possible to achieve discrimination between mcb rated 10 - 63 amp no matter what type you fit.
As an example a 16amp mcb will only have 'partial' dicrimination up to 500amps if fitted downstream of a 63amp mccb regardless of type but since most short circuits will be larger than 500amp it would require a front end mccb rated at 125amp to achieve full discrim'.
Now the example is using a MCCB upstream and a mcb downstream but when you get mcb covering mcb then discrimination is rarely achievable.