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

 

[spark 68]

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

Depends on the magnitude of the fault on the sub-DB circuit Shanky, with MCBs you only get partial discrimination at best, as Davesparks pointed out earlier HRC fuses > MCBs provide better discrimination.

 

[shanky887614]

Depends on the magnitude of the fault on the sub-DB circuit Shanky, with MCBs you only get partial discrimination at best, as Davesparks pointed out earlier HRC fuses > MCBs provide better discrimination.

Depends on the magnitude of the fault on the sub-DB circuit Shanky, with MCBs you only get partial discrimination at best, as Davesparks pointed out earlier HRC fuses > MCBs provide better discrimination.

what im saying though is the intake would be into a mainswitch that wouldnt trip then wither mcb's,mccb's idealy for each board.

they are all split off from the same point so how can the others trip honestly.

if the supply to all three was fed from the same mcb etc i could understand where you are coming from

 

[spark 68]

what im saying though is the intake would be into a mainswitch that wouldnt trip then wither mcb's,mccb's idealy for each board.

they are all split off from the same point so how can the others trip honestly.

if the supply to all three was fed from the same mcb etc i could understand where you are coming from

I agree with you here ^^, I thought you were talking about feeding the sub-boards from an MCB, my bad :35: lol

 

[shanky887614]

I agree with you here ^^, I thought you were talking about feeding the sub-boards from an MCB, my bad :35: lol

no, meter - main switch - x3 mcb's/whatever

thats just asking for trouble

 

[davesparks]

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

or mccb's

Yes BS88 boards are still available and have their place in installations. MEM excel boards spring to mind, but they don't have main switches so would put us back at square 1

 

[davesparks]

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

No it's not the same thing, you suggested installing a DB and feeding sub mains from MCBs in that DB.
That puts the final circuit MCBs on series with the distribution circuit MCB, this will may not discriminate in the event of a fault on the final circuit.

 

[spark 68]

No it's not the same thing, you suggested installing a DB and feeding sub mains from MCBs in that DB.
That puts the final circuit MCBs on series with the distribution circuit MCB, this will may not discriminate in the event of a fault on the final circuit.

That is what I thought I was replying to earlier, tis Friday night though

 

[davesparks]

You were indeed replying to that, it's post 9 where he said it.

 

[shanky887614]

i wouldn't use either i would use an rccb mainswitch like i was given and told to do on the last job XD

 

[paulthemagpie]

the installation is a 3 phase incoming, however only one the phases used which goes into a henly block then splits into 3.

so best practise would be to place the isolator before the henly block so the main switch covers the whole installtion.

oh and they reasons why the main switch cant be a RCD switch.

 

[Octopus]

the installation is a 3 phase incoming, however only one the phases used which goes into a henly block then splits into 3.

so best practise would be to place the isolator before the henly block so the main switch covers the whole installtion.

oh and they reasons why the main switch cant be a RCD switch.

Do you really need to ask this??

 

[paulthemagpie]

Do you really need to ask this??

do you realy need to ask that?

 

[Plonker 3]

Read 314.1 in the BGB then have a think why you shouldn't fit a RCD main switch on a TN system.

 

[Leesparkykent]

do you realy need to ask that?

Why not?

 

[paulthemagpie]

Read 314.1 in the BGB then have a think why you shouldn't fit a RCD main switch on a TN system.

potentially if we had nuisance triping it could be a killer for this type of business. so rcd protection as a main switch is a no no. RCBO protection for socket circuits is needed so only that circuit in question would disconnect instead of whole supply

 

[Deleted member 26818]

There is no requirement for RCD protection of socket circuits, unless the installation is considered a special location.
Just wondering whether the meter is single or 3 phase?

 

[shanky887614]

yes there is if its under 20amp supply. ill dig reg out of byb later

 

[Deleted member 26818]

yes there is if its under 20amp supply. ill dig reg out of byb later

Good luck with that.

 

[shanky887614]

Good luck with that.

 

[Deleted member 26818]

Thanks for that, as you can see from what you have quoted, there is no requirement for the circuit to be provided with RCD protection.

Socket-outlets are required to be provided with RCD protection, not the circuits supplying them, except in some instances where the installation is considered to be a special location, such as Horticultural or Agricultural.
In fact providing RCD protection for such circuits could be a non-compliance in some circumstances.

 

[shanky887614]

Socket-outlets are required to be provided with RCD protection, not the circuits supplying them, except in some instances where the installation is considered to be a special location, such as Horticultural or Agricultural.
In fact providing RCD protection for such circuits could be a non-compliance in some circumstances.

if the cable is not more than 50mm in wall or has mechanical protection then it needs an rcd

 

[Anthony S]

Exceptions still exist with a risk assessment

 

[shanky887614]

Exceptions still exist with a risk assessment

unless its a dwelling then no you cant skip it

 

[Anthony S]

unless its a dwelling then no you cant skip it

Who mentioned a dwelling ?

 

[Deleted member 26818]

if the cable is not more than 50mm in wall or has mechanical protection then it needs an rcd

Still no requirement for socket circuits to be provided with RCD protection.
Yes there are circumstances where it may be advisable, or the simplest solution to provide RCD protection for such a circuit.
However it still remains that there is no requirement in BS7671 for such circuits to be provided with RCD protection, except in some circumstances where the installation is considered to be a special location.
Doesn't matter whether there is a dwelling involved or not.

 

[Octopus]

do you realy need to ask that?

yes. Why not?

 

[Anthony S]

Still no requirement for socket circuits to be provided with RCD protection.
Yes there are circumstances where it may be advisable, or the simplest solution to provide RCD protection for such a circuit.
However it still remains that there is no requirement in BS7671 for such circuits to be provided with RCD protection, except in some circumstances where the installation is considered to be a special location.
Doesn't matter whether there is a dwelling involved or not.

411.3.3

Addition Protection RCD

Socket outlets not exceeding 20 A

 

[davesparks]

Ammendment 3 does not come in to effect until 1st July 2015.

 

[Deleted member 26818]

411.3.3

Addition Protection RCD

Socket outlets not exceeding 20 A

Yes socket-outlets, not the circuits supplying them.

 

[Leesparkykent]

The difference is the regulations say socket outlet and not circuit. For example you could have the circuit installed surface so doesn't require RCD protection but you would install a RCD socket outlet to comply.

 

[Guest666]

Ammendment 3 does not come in to effect until 1st July 2015.

No, it came into effect on the 1st Jan 2015 but the 'old' regs don't cease until the 30th June 2015 so the 3rd amendment is actually Currently in effect.

 

[Risteard]

No, it came into effect on the 1st Jan 2015 but the 'old' regs don't cease until the 30th June 2015 so the 3rd amendment is actually Currently in effect.

It was published on 1st January, with an effective date of 1st July. So BS 7671:2008 (2013) is still current.

 

[Leesparkykent]

No, it came into effect on the 1st Jan 2015 but the 'old' regs don't cease until the 30th June 2015 so the 3rd amendment is actually Currently in effect.

Wrong! this is of the IET website;


Amendment No. 3 to BS 7671 Wiring Regulations will be published on 5 January 2015, and will come into effect on 1 July 2015. The new reference for the book will be ‘BS 7671:2008+A3:2015’.
All new installations must comply with BS 7671:2008+A3:2015 from 1 July 2015.
In this update, a number of potentially life-saving changes have been proposed, which include, but are not limited to, changes to:
consumer unit enclosures (to come into effect January 2016);
wiring in escape routes;
changes to earth fault loop impedances for all protective devices;
the risk assessment approach for the omission of RCDs in non-domestic situations;
updated EIC and EICR forms; and
changes to definitions throughout the Wiring Regulations.

 

[Guest666]

Sorry.

 

[davesparks]

No, it came into effect on the 1st Jan 2015 but the 'old' regs don't cease until the 30th June 2015 so the 3rd amendment is actually Currently in effect.

Then there's yet another error in the book as it clearly states July!