RCD without neutral 2 phase supply

[humbayd]

Hi everyone,

Quick question can you fit an RCD without any neutral connections?

If I have a 3 pole circuit breaker which has all 3 poles terminated with a 3 phase supply (L,L,L excluding neutral or earth) on the primary side and I am using only 2 phases on the secondary side (L,L) to create a 400v supply to a transformer, can I connect an RCD to this breaker?

 

[newfutile]

I don’t think you can I have never seen a 3 pole (RCD) Residual Current Device and how could it work to detect earth faults which are single phase

 

[westward10]

The three pole is an mcb I assume, he wishes to connect an RCD from this?

 

[pc1966]

Yes you can normally use a 3-phase 4-pole RCD without neutral, or on just two phases, but check the manufacturer's instructions as often the self-test button is only between specific lines so those terminals should be used to ensure it works.

 

[pc1966]

I don’t think you can I have never seen a 3 pole (RCD) Residual Current Device and how could it work to detect earth faults which are single phase

The RCD just puts a sense transformer around all live lines used, so any imbalance is detected. As such the number of lines used has to be 2 or more so the legitimate return path is included.

Where it gets complicated is the self-test button on > 2 poles, and if the RCD needs a certain voltage on certain lines to function (i.e. more than just the energy of the fault current seen by the sense transformer). Type B RCD (DC sensing) need some form of supply for that aspect, but others like type AC/A often can trip on just a current.

 

[humbayd]

The three pole is an mcb I assume, he wishes to connect an RCD from this?

The distribution board has 3 phases coming into it and is terminated to a 3 pole circuit breaker, but on the secondary side we are only using two phases. So wanted to know if I can connect an RCD to monitor the 2 phases.

 

[humbayd]

Yes you can normally use a 3-phase 4-pole RCD without neutral, or on just two phases, but check the manufacturer's instructions as often the self-test button is only between specific lines so those terminals should be used to ensure it works.

Thanks for the response!!

Would this work with a set up of using a 3 pole breaker with 3 phases terminated on primary and only using 2 phases on the secondary, then connected a 4 pole RCD which only uses the two phases?

If the RCD senses load unbalance would this not end up tripping all the time as we are only using the two phases of the three?

 

[LewisM]

I don’t think you can I have never seen a 3 pole (RCD) Residual Current Device and how could it work to detect earth faults which are single phase

I’ve used a 4 pole RCD with just the three phases connected and it works fine just the test button doesn’t work

 

[pc1966]

Would this work with a set up of using a 3 pole breaker with 3 phases terminated on primary and only using 2 phases on the secondary, then connected a 4 pole RCD which only uses the two phases?

It should do. It might be better to put all 3*L from MCB to RCD if they are close together, just in case it has any future use.

If the RCD senses load unbalance would this not end up tripping all the time as we are only using the two phases of the three?

The RCD only cares that the sum of all currents is close to zero. So if L1 is the exact opposite of, say, L2 then the sum is zero and nothing trips. If L1 leaks to E or anything else, then L2 != L1 and if big enough difference it trips.

With a 3P only load like a motor, the current from L1 returns via both L2 and L3, etc. With a 3P+N load then you could have different SP loads from Ln to N, etc. In all cases what matters is the vector sum of L1+L2+L3+N is close to zero. If L3 and N are zero themselves, then you need L1 = -L2 which, if wired normally and all OK, is the case.

 

[pc1966]

I’ve used a 4 pole RCD with just the three phases connected and it works fine just the test button doesn’t work

I think it depends on the make/model of RCD. Some have the test button diverting L1 out to N in, others go L1/L2 or L1/L3

 

[Rockingit]

I’ve used a 4 pole RCD with just the three phases connected and it works fine just the test button doesn’t work

The only problem with that is that it then fails inspection criteria! With so many main manufacturers making 3p RCD/RCBO devices I just can't see a reason to not use the right device for the job.

 

[loz2754]

How about replacing the MCB with a 3 phase RCBO?

 

[nicebutdim]

How about replacing the MCB with a 3 phase RCBO?

Not commonly stocked in wholesalers. Most here would hand out a 1ph 10kA RCBO if asked for one.

 

[pc1966]

If the DB has a matching 3P RCBO at a sane price that is the simplest solution.

Also fitting the 3P MCB and 4P RCD as if a full 3P+N load is needed, and just using Lx & Ly for the single 400V load is perfeclty reaonable and ensures the RCD works as intended. If in the future you need any other use, it is ready to go.

 

[humbayd]

How about replacing the MCB with a 3 phase RCBO?

I think that might have to be one of the options. Thanks