Contactors tripping RCD

[BigBill53]

Hi,
I have an old resistive starter that has been overhauled. We installed 8 new 250A contactors, all with 240VAC coils. When we were testing the starter the main RCD to the board would trip when the third contactor turned on. It should be noted that we did not have the motor connected at this time - we were simply testing the starter to see if all the contactors came in at the right time.

Well, we didn't really know why this was happening so we jacked up the settings on the RCD temporarily (up to 1000mA for 3 seconds). Now we can get the 7th contactor to come in before the RCD trips.

This has me baffled. Why is the RCD tripping? Everything runs fine when the RCD isn't connected. We have to leave the RCD connected though because of a directive from the mines inspector (I won't get into that now though).

 

[ScottyUK]

Bad RCD?

Do the contactor coils have suppressors by any chance?

 

[jraef]

Are all of your coils coming off of the same circuit? I ask because an RCD is looking at an imbalance in phase currents, and the magnitude of the current spike when the coils pull in might be creating a severe voltage drop on that one phase, which the RCD interprets as current not returning and it trips. It might also be indicative of the control circuit wires being too small for the cumulative coil loads, especially since the problem exacerbates as the number of coils increases.

"Will work for (the memory of) salami"

 

[waross]

Or a failing coil, or a fault in the wiring to one contactor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crthompson]

What kind of CT configuration are you using? Is it truely residual? You may want to go to a zero sequence current sensing CT. You may also want to look into the residual properties of the relay that is tripping (if your sensing is truely residual). I have ran into problems when the manufacturer offers different sensing elements for residual tripping. Is this a longwall mining application?

I am an engineer, so the glass is twice as big as it needs to be.

 

[BigBill53]

Thanks, I will have a look into that. You never know sometimes.

After looking more closely I suspect it may be related to a bad installation. Because they had to fit this to an existing board, they have used a huge CT to fit around the busbars, and one of the phases is much closer to the inside edge of the CT while the other two are in the middle.

At this point we have just turned the RCD off. It's actually not in our standards or regulations that we need to have it on this circuit.