CNC machine insulation testing with a megger

Hi guys, it's been a long time since I have posted to RCM, I mostly lurk.
I come begging for some of the group's wisdom.
I have a friend who has purchased an old Mori Seiki SL-3B Lathe with
Fanuc 5T control for his high school shop. Since it was made in Japan
many years ago it did not come with UL or CSA approvals, something that
is required before it can be hooked up in the school. (BC Canada BTW) The
normal procedure for imported machinery is to get an approval sticker
from the local electrical inspector.
He has done a preliminary inspection and stated that he requires an
insulation test of 2x the supply voltage plus 1000v for one minute,
during which time he checks for insulation breakdown and listens for
arcing. I assume he will be using a megger. We're a little reluctant to
allow 1400 volts to be applied to the input, parts for this age machine
aren't easy to come by. He said boards may be removed during the
testing, but that still leaves the DC spindle drive and servo power
supplies. I do not know if the rectifiers can handle that much voltage.
Is this a reasonable method of testing what basically amounts to a
computer? Is it going to kill some part of the machine?

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This is a standard, easy to pass test called a Hipot Test. The current is limited during the test so the chance of damage is small.
The voltage is applied between ground and the neutral/hot conductors. The diodes should not see it as an overvoltage as they are either on the secondary of the line transformer, or not connected to ground.
If the unit has built-in surge protectors or varistors connected directly to the AC input, these should be removed before the test and then reinstalled. If they are left in, they will conduct (as they should) and cause the unit to fail the test.
Hipot tests are done to most computer power supplies.
Reply to
Jim Stewart
I was involved about 15 years ago in a similar situation in Newfoundland. We were installing an air conditioner that for some reason was not CSA approved. The inspector showed up with a specialised test set that generated an adjustible DC, current limited, high voltage. Basicly he hooked this thing on & cranked up the voltage until you could hear a hissing noise from the coronal discharge. The voltage & leakage current met specs so we were good to go.
Are the DC portions of the controler fed directly from the mains or are they off a transformer? The semiconductors for sure will not take the test voltage if they are on the mains but the I believe as it's a DC tester anything on the secondary side of a transformer should be O.K., any leakge is more likely to go to ground IN the transformer than go through the secondary winding. If it worries you that much it should be acceptable to the inspector to jumper the secondaries directly to ground with alligator leads, that wouldn't affect anything on the primary/mains side which is what he's really interested in. Remember, what they are checking is that the mains power and/or line spikes arn't likely to end up anywhere they shoudn't, i.e. in you :(.
If the motors are fed from a rectifier block fed directly from the mains he might be ameniable to removing the diode block/controller/etc. & testing the motor(s) for leakage individually.
Still raining?
Howard, in Balmy Halifax.
Reply to
Howard Eisenhauer

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