Grounding a dc power supply on the negative side...what's the purpose? 2

[bdn2004]

A DCS vendor has sent out an instruction that all of the 24VDC power supplies should have their negative terminal connected to ground.
I think they are marked 24V+ & 24V-. This system up/running no issues for a couple years now. What would be the purpose of such an instruction?

 

[ScottyUK]

Hi Alan,

Most turbine-generators have a dedicated battery whose primary purpose is to power a pump sufficiently large to provide lube oil to the bearings for long enough to get the machine safely to a standstill in the event of a total loss of internal power. These pumps are usually a fair size, especially for a relatively low voltage DC machine, so the armature currents are correspondingly large. Batteries are typically 110V, 125V or 220V and from 1000 Ah - 3000 Ah capacity with the larger AH ratings at the lower voltages. It's quite common for other smaller loads such as protection relays, control equipment, excitation field flashing, switchgear local to the machine, and so on to tap off the battery too.

 

[submonkey]

Very interesting - Thank you Scotty!

 

[ScottyUK]

I should also have included among those critical loads a seal oil pump to keep the hydrogen coolant in the generator for long enough to vent it in a controlled fashion rather than have it escaping past the shaft glands into the machine hall.

Glad the info was of interest.

 

[roydm]

I have seen systems where the system was ungrounded, two lamps in series with the center point grounded so if either line developed a fault one lamp becomes brighter, it's fine but very hard to find a fault. A grounded negative system is much easier to locate a fault on as it will bvlow a fuse or trip a breaker.