splitting across 3 phases

kelledy

Could anyone confirm for me.... If you have a 3phase d/b with single phase skt circuts and 3ph skt circuts and they need to be split across 3 phases, can a 3 ph rcd protect single phase circuts in order to evenly distribute them across the 3 phases?
This is for an exam .
Thanks

2011

kelledy wrote: »

Could anyone confirm for me.... If you have a 3phase d/b with single phase skt circuts and 3ph skt circuts and they need to be split across 3 phases, can a 3 ph rcd protect single phase circuts in order to evenly distribute them across the 3 phases?
This is for an exam .
Thanks

Not permitted.

kelledy

2011 wrote: »

Not permitted.

so am i right to say that if there are a small number of single phase circuts that need rcd protection .... I think theres 3 that this would be ok on one phase? Using single phase rcd? Then thats not splitting the load????

kelledy

2011 wrote: »

Not permitted.

Can you tell me where you sourced dis info as i have lads telling me there is no problem protcting a single ph mcb off a 3 ph rcd using 3ph busbar? Experienced senior electricians .

2011

I would install an RCBO per circuit. Single phase circuits should be evenly distributed across the 3 phases so much as is practical.
I would normally select 16A type C RCBOs for an industrial installation with the CPC and phase conductor the same size (generally a 4 mm sq.)

2011

kelledy wrote: »

Can you tell me where you sourced dis info as i have lads telling me there is no problem protcting a single ph mcb off a 3 ph rcd using 3ph busbar? Experienced senior electricians .

I had a quick look at the regulations to find it, but could not this.

The fact that you have a 3 phase supply suggests to me that you are dealing with a commercial or industrial installation. Standard practice for such installations is to install an RCBO per single phase socket circuit. If a 3 phase RCD (protecting several 1ph sockets circuits) were to trip it would mean that more than one circuit would be lost. Generally this would not be acceptable with a commercial or industrial installation.


I accept that a 3 phase RCD would save a few bob, but in the real world this is not what is done in industrial installations. We typically look at the worst case socket circuit (longest run) and size a cable for this. All similar circuits use the same cable size and protective device. The RCBOs would always be installed in multiples of 3 (as there are 3 phases). We would normally allow for 25% future expansion, so each protective device type would have a minimum of 25% spares installed as standard.

Calgary22

could you not have one phase feed into a single phase rcd protecting the mcbs that require a rcd protection one phase feeding lights and one phase feed other items such as pump that also require no rcd?

example 3 phase comeing in through 3 phase main switch fuse. from l1 feed lights from l2 feed cooker and 2 other mcbs then l3 into bottom of single phase rcd? feed out of top of rcd to sockets etc that require rcd protection.

I agree 2011 but this not a real world job:D and the runs would only be a couple of meters its only a small work area. and also there would not be enough rcbos.

2011

Calgary22 wrote: »

could you not have one phase feed into a single phase rcd protecting the mcbs that require a rcd protection one phase feeding lights and one phase feed other items such as pump that also require no rcd?

You could but:

1) Loss of a phase would result in loss of all sockets or all lights as opposed to 1/3 of the lights or sockets.

2) The phases are less likely to be well balanced.

3) It would be considered bad practice.

Calgary22

your right 2011 i am thinking 2 single phases rcd and 1 rcbo or should i just go the whole hog and use 6 rcbos:D

Calgary22

this whole process is extremely head wrecking but its great to be able to do it usually its all done for us:D

kelledy

2011 wrote: »

I had a quick look at the regulations to find it, but could not this.

The fact that you have a 3 phase supply suggests to me that you are dealing with a commercial or industrial installation. Standard practice for such installations is to install an RCBO per single phase socket circuit. If a 3 phase RCD (protecting several 1ph sockets circuits) were to trip it would mean that more than one circuit would be lost. Generally this would not be acceptable with a commercial or industrial installation.


I accept that a 3 phase RCD would save a few bob, but in the real world this is not what is done in industrial installations. We typically look at the worst case socket circuit (longest run) and size a cable for this. All similar circuits use the same cable size and protective device. The RCBOs would always be installed in multiples of 3 (as there are 3 phases). We would normally allow for 25% future expansion, so each protective device type would have a minimum of 25% spares installed as standard.

This is a fas based exam for award of craft diploma. Rcbos are not provided but installation must be balanced. Single phase rcd in my opnion will not balance the loads as you would have 2/3 circuts on the single phased rcd. Im am looking more towards balancing everything off the 3ph rcd and 3ph mcb mains.
I am unsure weather this is good/bad practice. This is just a small commercial installation with few circuts so i am thinking it might be best. What do you think?

It seems to be accepted in the regs.
Also would a single ph circut protected by 3ph rcd .... Would this have an effect on the reading when testing of the sockets?

2011

kelledy wrote: »

Rcbos are not provided but installation must be balanced.

If there are no RCBOs no problem, use RCDs. This has nothing to do with balancing the load across 3 phases.

Single phase rcd in my opnion will not balance the loads as you would have 2/3 circuts on the single phased rcd.

I don't understand this statement.
Normal and good practice (if no RCBOs are available) is to install the 1st socket circuit on L1 supplied from a 1 Ph RCD & MCB, the 2nd socket circuit on L2 supplied from a 1 Ph RCD & MCB, the 3rd socket circuit on L3 supplied from a 1 Ph RCD & MCB, the 4th socket circuit on L1 supplied from a 1 Ph RCD & MCB.......... and on it goes.
This will balance the loads as best as is practical.
The tripping of any single device (MCB or RCD) will have minimum impact on the installation.
For many businesses down time is very inconvenient and costs money, this is the driver.

Im am looking more towards balancing everything off the 3ph rcd and 3ph mcb mains. I am unsure weather this is good/bad practice.

This is bad practice.
Were the to RCD trip then a number of downstream socket circuits would be lost, rather than just one.
In addition, the more circuits on a single RCD the higher the likelihood of nuisance tripping.
As stated above this is not how it is normally done a real world situation.

It seems to be accepted in the regs.

Yes, you appear to be correct.
Meeting the minimum standard as outlined in ET101 and best practice are often two different things.

Also would a single ph circut protected by 3ph rcd .... Would this have an effect on the reading when testing of the sockets?

No effect, except if I was marking your exam it would cost you marks :D
I am sure that many installations do this.
The only advantage is a small cost saving.

Bruthal

I was going to mention that the more socket circuits on an RCD, the higher the risk of cumulative leakage and nuisance tripping. And a 3 phase RCD could end up with a load of single phase socket circuits. 2011 has it covered though.

Also, a neutral to earth fault could be harder to quickly track down when the 3 phase RCD trips out a load of single phase socket circuits.

No matter how well you balance the socket circuits over a 3 phase RCD, they can be balanced exactly the same way with single phase rcd 's, just a little more expense though.