three phase rcd

[markythesparky]

Hi guys.

Can anyone explain in simple-ish terms how a three phase rcd measures a currents imbalance? It can't be between phases as not all three phase systems are balanced equally so it must be between phase and neutral big how?

Just curious!

 

[Marvo]

It measures the same as you would clamp a 3 phase circuit with your earth leakage clamp meter to test for leakage. If it's 3 phase no neutral then the sum of all the currents flowing simultaneously in the 3 phases will equal zero if there's no leakage. Likewise with a 3 phase and neutral circuit except the neutral current must also be included in the zero sum equation.

Current in will always equal current out.

 

[Farmelectrics]

interesting if its not a balanced load then it will need a neutral so yes any in balance between line and neutral would trip the rcd but im interested myself to here other peoples versions of how a balanced load with no neutral would trip the rcd

 

[Farmelectrics]

sort of understand what marvos saying

 

[Farmelectrics]

marvo if say there was 10amp on each phase so 30 amp total how would this equal zero ive had this explained to me a while ago but scratching head trying to remember lol

 

[telectrix]

exactly equal currents in each phase means the the current in the neutral is Zero

 

[Marvo]

marvo if say there was 10amp on each phase so 30 amp total how would this equal zero ive had this explained to me a while ago but scratching head trying to remember lol

Because the voltage phase angles of each phase are 120 degrees apart so when current is flowing one direction in one of the phases it's flowing in the opposite direction in at least one of the other phases.

Current flow in a circuit is a physical thing like water flow in a pipe, an Amp by definition is a Coulomb of electrons flowing past a point in one second and a Coulomb is an actual number of electrons. You can picture an Amp of current flow as being similar in nature to a litre per second of water flow in a plumbing circuit so if a quantity of electrons (current) is flowing one direction in one leg of a circuit an equal quantity of electrons (current) has to be flowing out of the circuit.

 

[Farmelectrics]

thanks marvo wow your first explanation was easier ha ha cheers

 

[Lucien Nunes]

An RCD can theoretically work on any number of conductors, so long as every conductor in the circuit has its own sensing winding on the RCD transformer. A single-phase RCD has two windings, a 3-phase has four, you could make one with six windings for 5-phase power!

Farm, visualise a 3-cylinder radial aero engine, with all three con-rods sharing a crankpin. The positions of the pistons represent the currents / voltages on a balanced 3-phase system - the moving mass of each piston is balanced by the sum of movements of the other two.

 

[Marvo]

thanks marvo wow your first explanation was easier ha ha cheers

Lol, I'm trying to keep it simple, honest.

If you understand how a single phase RCD works then you should be able to get how a 3-phase RCD does. It's basically a current transformer where current through the circuit conductors all cancel each other out so they balance which means that no current is induced into the sensing winding. It's only when there's leakage to earth that they no longer balance and it's this imbalance that causes a current in the sensing winding which in turn operates the mechanism that causes disconnect.

 

[ElectroChem]

My tradesman got caught out by one of these the other week. Scratching his head why the 30A 3-phase drives were running fine but a piddly little burner tripped the 200A main switch every time. Turns out another worker had left the neutral out of the main RCD.