Earth Leakage Toroid Systems.

THANKS GURU for the answer, very useful information ,simple and easy to understand

If, as you state, the toroid is mounted at the main switchboard, and said generator is supplying that switchboard, then I fail to see how that toroid and associated equipment will protect the cabling between the generator and switchboard, particularly if the generator is somewhat remote from the switchboard.

Not knowing the relative positions of the generator and the main switchboard, it's not possible to give a definitive answer, but be aware that the earth leakage relay will only protect against earth faults downstream of the detection toroid. Therefore, to protect the generator to switchboard cabling, it should be mounted as close to the generator as is possible but downstream of the MEN link.

All 3 phases plus the associated neutral must pass through the toroid for the device to work correctly.

The other question that comes to mind is the location of the main switch that the relay disconnects. If it is on the main switch board then it is not actually disconnecting an offending generator supply cable. The disconnection should take place at the generator.

Wow that simple, don't I feel a little silly. There is so much to learn about this electrical game we play.

Thank you for the prompt replies & links.

Sorry I did not make my question that clear.

We have an existing toroid mounted at the switchboard about 10 metres away, the new one we have installed is located inside the generator where the windings are brought out and all the control gear is located behind the generators control panel, they were kind enough to leave room for extra components.

Its funny you should mention that it has to be downstream of the MEN, the reason I started to find more information is we have done 2 different generators this week and cannot work out why one of them does not trip on an induced fault, if we use the test function on the relay it shunts out the main switch but using 500 or 1000mA test from our RCD tester we cannot get a trip time.

My boss will be pleased to know we have to re locate the coils, thanks a lot I believe you solved our problem.

A test function on most RCDs and sometimes on relays is normally only a mechanical test to ensure that the mechanism is working correctly, it does not necessarily test the actual capability of the device to detect and operate on an actual earth fault.

Is it possible that your second generator does not have an earth link, or that it is downstream of the RCD/toroid, either of the above will cause the RCD to not trip on an earth fault.

A generator is basically an isolated supply in that it has no winding to earth connection (no current path to earth). So, until you make that connection by either earthing the neutral in the case of a star point winding configuration, or one of the live conductors in the case of a single phase generator (there are a few other setups but we'll leave them alone), there is no path for stray earth currents to return to the generator and hence no chance of the RCD detecting such stray current.

Obviously the neutral - earth connection has to be upstream of the RCD so that the return earth leakage current bypasses the RCD and therefore creates the out of balance that causes it to trip.

Perhaps I should elaborate a little on my previous comment "So, until you make that connection by either earthing the neutral in the case of a star point winding configuration, or one of the live conductors in the case of a single phase generator (there are a few other setups but we'll leave them alone), there is no path for stray earth currents to return to the generator and hence no chance of the RCD detecting such stray current."

Actually, with a "first fault" situation there will be no stray current to detect as there will be no current flow to earth and no live conductor to earth shock hazard either.

It is only upon the occurrence of a subsequent fault that the danger applies.That second fault normally needs to be in a different live conductor to the first fault for there to be a shock hazard, but there are exceptions.

I follow you and understand 100% now, we know both generators have MEN links via bus bars mounted under the main switch of then gen's.

We mounted the toroids upstream of the links so any fault currents will just re join the neutral current then back through the toroid. What a stupid mistake which will eat 4-6 hours to fix them both up.

If I had any idea how it worked before we installed it then I would have thought of that myself, how annoying.

Consider that all the toroid is doing is measuring out of balance currents, therefore you may be more easily able to fix the downstream link problem by passing the main earth conductor from the earth bar through the toroid to the downstream neutral-earth connection. It doesn't matter which way the earth wire goes through the toroid as any current in the earth conductor will either add to or subtract from the total active value and will create the out of balance necessary to trip the breaker.

It is quite common for a multicore cable with included earth conductor to be passed through the toroid and then to return that earth back through the toroid to negate its previous passing and thus allow the RCD to correctly detect downstream earth faults.

It is not the task of an earth leakage device to detect imbalance. The clue is in the name.