[fogassa]

Im retrofiting a votan cnc machine, im trying to use the existent controll panel (box) and all the dc control wiring that includes all of the machine relays,solenoids etc exept for the ac stuff.
problem is that when i test for continuity between the dc negative rail and earth ground i get a dead short.
is it normal or does it mean there is some component (rele,solenoid etc.) that has a short to ground.

[Jim Dawson]

It may or may not be OK.

If the DC power comes from a transformer that isolates it from the AC supply, it is possible that one leg of the transformer secondary is earthed. This is a common way of doing it.

If the DC power is directly rectified from the AC line with no transformer isolation then there is a problem.

[fogassa]

there was a isolating PS , but it was a 380v input so i have exchange the original Ps by a chinese 24vx14 amps Ps and installed a isolation transformer to the mains input of the chinese Ps.
the thing is that without the power supply conection to the 24v rails there still be a short between negative and ground on the wiring, not the power supply.

[Jim Dawson]

It is possible that the DC bus negative rail is grounded at the bus. Look for a green/ yellow wire connected to the DC negative bus

[fogassa]

Right on it! any reason for that?

[Jim Dawson]

This is very common, especially on European machines. Both grounded or ungrounded works, but I always ground the DC bus negative just to keep everything referenced to earth ground. Seems to help with electrical noise. There are some instances that require isolated DC, but these are not common.

[fogassa]

you are the man. so i´ve this chinese power supply that shares the common with the - output, it is not isolated but i´ve used a isolation transformer on the power supply mains input. i dont think there will be a problem with the negative of the bus connected to the earth or there will be? thanks btw.

[Jim Dawson]

I think it will be OK, I have a Chinese 24V power supply wired that way on one of my machines, has been working fine for the last year or so.

[fogassa]

Thank´s a lot.