main switch question

[paulthemagpie]

Hi guys,

Im installing some main switches in an installation to cover 3 boards.

A question i have is, they anything against the regulations or indeed bad practice to fit a single main isolator switch to cover the 3 boards or should they have 3 individual ones.

What i got is a single phase load coming out main supply into a henly block which then feeds 3 DBs. Should it be okay to pick the incoming feeds into the henly block and take then into isolator and then new meter tails from isolator back into the henly block which would then feed the 3 dbs. So basically putting the main switch before the henly block?

obv this is the cheaper and quicker solution, but i just want to make sure im not missing anything which makes this against the regulations?

thanks in advance

 

[bigdavs83]

What's on the three boards? How big is the fuse? I would probably be inclined to fit three isolators.

 

[paulthemagpie]

I havnt actually seen the installation, yet so got limited info. On a pic i seen the other day it looks like BD1 and BD2 are main switches feeding boards else were in the building so it might just be a case of the main switch is only needed for DB3.

However if thats not the case then the other 2 boards only very small. maybe a one way and and 3 way. not sure what they feeding as of yet.

 

[davesparks]

An installation should have a single main switch which isolates the entire installation.

 

[Octopus]

I havnt actually seen the installation, yet so got limited info. On a pic i seen the other day it looks like BD1 and BD2 are main switches feeding boards else were in the building so it might just be a case of the main switch is only needed for DB3.

However if thats not the case then the other 2 boards only very small. maybe a one way and and 3 way. not sure what they feeding as of yet.

I'd be going to have a look!

 

[telectrix]

posts #4 and #5 are your best advice.

 

[shanky887614]

install a board and feed the others off this, much neater.

 

[davesparks]

install a board and feed the others off this, much neater.

But not necessarily the right answer, depending on the type of board used the main switch may not be suitably rated for the switching duty or it may not be possible to achieve discrimination in the protection.

There has been no mention of the size of the incomer or main switch or the nature of the installation. It could be an 800A TP supply for all we know

 

[shanky887614]

But not necessarily the right answer, depending on the type of board used the main switch may not be suitably rated for the switching duty or it may not be possible to achieve discrimination in the protection.

There has been no mention of the size of the incomer or main switch or the nature of the installation. It could be an 800A TP supply for all we know

???? a main switch doesn't trip,

its a single phase supply i would be surprised if its over 100amp because above this i would have thought it would be 3 phase.

how would discrimination be a problem if 3 mcbs feeding 3 sub boards?

if thats true then every large building in the country has been wired wrong.


thats one big henley block for 800amps then lol.

busbar chamber with isolator for each if thats the case then

 

[davesparks]

???? a main switch doesn't trip,

its a single phase supply i would be surprised if its over 100amp because above this i would have thought it would be 3 phase.

how would discrimination be a problem if 3 mcbs feeding 3 sub boards?

if thats true then every large building in the country has been wired wrong.


thats one big henley block for 800amps then lol.

busbar chamber with isolator for each if thats the case then

I didn't say anything about the main switch tripping, I mentioned switching duty which is the type of load it is able to switch. If the load is highly inductive it needs a different switching duty to a standard DB main switch.

I'm sure he said earlier that the 2 pole isolator suggested was no good as he had a three phase supply.

Discrimination will be a problem if the tripping curve of the sub main MCB overlaps that of the final circuit MCB in the sub DB. Discrimination is concerned with ensuring that under fault conditions the final circuit protection will operate before the distribution circuit protection. This is one of the reasons HRC fuses or MCCBs are preferable for submains.

Yes I missed the bit about the Henley blocks, that suggests it's going to be no bigger than 200A

 

[shanky887614]

I didn't say anything about the main switch tripping, I mentioned switching duty which is the type of load it is able to switch. If the load is highly inductive it needs a different switching duty to a standard DB main switch.

I'm sure he said earlier that the 2 pole isolator suggested was no good as he had a three phase supply.

Discrimination will be a problem if the tripping curve of the sub main MCB overlaps that of the final circuit MCB in the sub DB. Discrimination is concerned with ensuring that under fault conditions the final circuit protection will operate before the distribution circuit protection. This is one of the reasons HRC fuses or MCCBs are preferable for submains.

Yes I missed the bit about the Henley blocks, that suggests it's going to be no bigger than 200A

fair enough but im sure you can still buy bs88 boards.

or mccb's

 

[HandySparks]

...What i got is a single phase load coming out main supply into a henly block which then feeds 3 DBs....

Do you really mean that you have three single phase feeds (each on a different phase), derived from one three phase incoming supply?

 

[7029 dave]

So single phase supply, with multiple switch gear, one point of isolation between meter an switch gear go for it.

 

[Risteard]

how would discrimination be a problem if 3 mcbs feeding 3 sub boards?

An MCB feeding a distribution circuit for a dis board with MCBs on it is nearly always going to suffer from a lack of discrimination/selectivity. Cascading MCB discrimination is nearly impossible to achieve.

 

[shanky887614]

An MCB feeding a distribution circuit for a dis board with MCBs on it is nearly always going to suffer from a lack of discrimination/selectivity. Cascading MCB discrimination is nearly impossible to achieve.

an mccb main switch then mcb's or mccb's feeding sub boards.

when the shower/sockets/lights trip it doesnt trip every other mcb does it, same thing