Lathe trips RCD on Generator

I have a Herbert Senior No4 preoptive lathe (early 1950's). Was working perfectly until we moved house. The lathe is 3 phase and now is connected to a 3 phase generator (110KVA). As soon as the main switch on the lathe is turned on ( not the motor) the RCD kicks out. I have put a meter across the terminals of the lathe plug and shows 306 ohm between phase 1 and earth, 306 ohm between phase 2 and earth open circuit between phase 3 and earth. 136 ohm between ph1 and ph2. Has the move dislodged something internal? are these numbers acceptable and the RCD needs to be bypassed? Could there be a short in the lathe?
Any help greatly appreciated.

Look for a loose connection on leg 3. If you don't find the open, check that the generator is actually producing all 3 phases.

Does the generator voltage match the voltage that the lathe is wired for?

Flipping the main on should only be putting power to the control transformer. Sounds like there is a mismatch in the voltage configuration of either the generator which is new from the other location or the lathe. Just need to make sure both match.

dbooth said:

I have a Herbert Senior No4 preoptive lathe (early 1950's). Was working perfectly until we moved house. The lathe is 3 phase and now is connected to a 3 phase generator (110KVA). As soon as the main switch on the lathe is turned on ( not the motor) the RCD kicks out. I have put a meter across the terminals of the lathe plug and shows 306 ohm between phase 1 and earth, 306 ohm between phase 2 and earth open circuit between phase 3 and earth. 136 ohm between ph1 and ph2. Has the move dislodged something internal? are these numbers acceptable and the RCD needs to be bypassed? Could there be a short in the lathe?
Any help greatly appreciated.

Click to expand...

There is your problem.

Now you need to find WHY you have that.

Was it connected through an RCD before?

I am assuming that your lathe is not provided with a neutral connection, that you just have 3 phase wires and an earth going to it. With your 240V mains, measured from neutral to line, a 300 ohm resistance to earth will draw a good part of an ampere, which is more than even most industrial RCDs will allow.

I do not know for sure what caused your problem. You are going to have to discover that because you are the guy on the spot with the ability to look and test.

However, I do not think you have a classic "short circuit" on the mains. It seems to be that some other wire is connected to earth, something with a resistance of around 300 ohms. And it may not be the motor, although not knowing your setup, I cannot be sure.

Presumably, in the condition you have, the motor is not connected to the plug, since the switch is not in the "on" position. But not all setups totally disconnect the motor, although that is usually required these days.

Something may be connected from line to ground, possibly because whatever it is cannot be set for your 3 phase voltage. A light, a transformer, a relay of some sort, etc.

If your unit DOES receive a neutral connection (which requires a 5 wire plug), see if current on the neutral can trio the RCD. it really should not do so, neutral current is not a "fault", it is just evidence of an imbalance.

You say "is now connected" to a generator.... if it was not previously hooked to that, maybe the "problem connection" has existed for a long time, and only shows up with the genset and associated RCD.

Hi dbooth. I am assuming at the house you moved from you had this Lathe connected to a 415 volt 3phase service. Very few houses in Australia would have R.C.D protection on a 3phase circuit in a domestic situation, so I am assuming this machine was not previously connected through a R.C.D and now when you switch it on through a R.C.D (your generator) it immediately trips.
Would you confirm my above assumptions and does this machine have 3 actives and earth or 3 actives, neutral and earth, If it does not have or use a neutral it's control circuits will operate at 415v or it may have a control transformer which might have it's primary side connected to 2 of the actives getting 415v or if it is connected with neutral and one active it would supply the control circuits with 240v.
Our R.C.D's are mostly 30mA. In hospitals, for example they are only 10mA. The R.C.D's job is to protect people not installations.
Could you post some pictures of inside the Lathe electrical control cabinet and associated electrical equipment.
My thoughts are if it has a control transformer start here, but you have resistance showing up where it should not be, so start disconnecting things and you will soon find where this resistance is coming from.

Jim