3 phase rcd

[Maxxy]

Hi all. Does anyone know how these work?

Basically i was on a job today (pipe fitting, not elec) and they had used isolators, then dol starters/overloads, and finally an rcd before a 3 phase commando outlet.

Do 3 phase RCD's measure the current down each phase and compare it, rather than comparing it between the live and neutral?

There was no neutral connected to the supply of rcd, but there was from the rcd to the commando socket.

 

[Spark123]

Yes, in a 3 phase circuit the sum of the currents in a healthy circuit passing through the RCD equal zero, (i.e. for a simple 3 phase motor at a single point in time if L1=1A then L2= -0.5A & L3= -0.5A). If for whatever reason these do not equal zero it indicates there is a fault. If this fault current exceeds the trip point of the RCD it will trip.

edit to clarify

 

[Maxxy]

Ah i see! Why did the RCD have a neutral terminal though? Or is that incase the 3 phase isnt balanced...in which case it just makes sure everything going in is coming back out?

Just seemed a bit weird to me the fact that the supply to the rcd didnt have a neutral but the load to the plug did.

 

[Spark123]

I don't follow? All the live conductors in the circuit need to pass through the coils of the RCD, if it is a 3wire 3phase then only the 3phase conductors need to pass through the RCD, i.e. can be normal in the cases of a 3phase motor or transformer.
On the other hand, if the supply isn't being used to supply 3phase equipment or is being used to supply a load where a neutral is required, the neutral (which is a live conductor) must also be wired through the RCD. Similarly, if a 5pin commando socket is provided then the neutral must also be provided. If a 4pin commando socket is provided then the neutral is ommitted as only appliances which do not require a neutral can be plugged in.

 

[bernardgreen]

For a Delta connected load there is not neutral so any significant earth leakage would trip an RCD with its sensor summing the three phases.

For a Star connected load there is a neutral and this has to be included in the summation of currents in the RCD.

But also for connected Star connected loads which should remained balanced a current sensor may be fitted on the neutral to detect out of balance and shut down the power.

 

[ColJack]

star connected loads also do not have a neutral if they are ballanced..

the ONLY time you need a neutral is if the loads are unballanced, or you require a single phase from the supply.. (which usually unbalances it anyway.. )

 

[ricicle]

If you are fitting a 3 phase RCD on a load which you know has a single phase tap off then you need the neutral as one phase will be passing current and it needs to see the same current pass through the neutral as well to ascertain that there is no leakage current as no current is passing in the other 2 phases

 

[Maxxy]

Cheers for the replies! Yours was rather good spark123, covered it nicely. I understand star delta etc, just didnt know how the rcd worked with 3 phase except for a bit of an educated guess.

It was just one of those things you see, and it plays on your mind until you find out the answer!

 

[plugwash]

a couple of things that haven't been mentioned

1: if the socket has a neutral (as it sounds like the original posters does) it damn well should be connected back to the mains (though the RCD if there is one), otherwise if someone plugs in equipment that needs a neutral it could be damaged.

2: some RCDs need the supply neutral terminal to be connected for the test button to work, i belive MK lists a workaround for this issue on thier RCDs though so if your test button doesn't work check the manufacturers documentation.

 

[Maxxy]

a couple of things that haven't been mentioned

1: if the socket has a neutral (as it sounds like the original posters does) it damn well should be connected back to the mains (though the RCD if there is one), otherwise if someone plugs in equipment that needs a neutral it could be damaged.

That was why it confused me, because it wasnt. I did mention this to them, but as i was only there as a pipe fitter, not a sparky they may not have paid much attention.