Common Motor Control Power Transformers -Grounded or floating.

[rcw retired EE]

I'm helping on an IEC project in Africa and want to know if ungrounded control power is typical for IEC MCC's. MCC's were built in China.

Each 690V MCC has a 690V-230V, single-phase control power transformer that feeds a distribution panel with single pole breakers, one breaker per MCC bucket. The 230V winding is floating ungrounded on four of the transformers and is grounded on two of them. One has the hot leg grounded so the control circuit is still energized when the CB is open. The two grounds appear to be unintentional, (haven't located them yet).

I told the electricians to find and fix the unintentional grounds and then ground the neutral side - the one not connected to the breakers.

Is there a good reason to leave the control power ungrounded? I couldn't come up with one. BTW there is no ground detection circuit.

 

[ScottyUK]

I would say it's unusual to have the control power ungrounded. If ungrounded it should be a double pole MCB. the only reason for using an ungrounded supply would be to allow continued operation with an earth fault on one pole. I'd prefer to see 110V AC than 230V for a control voltage.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

I had one experience with an ungrounded control system. Difficult to troubleshoot is an understatement. We would put an intentional ground on the control transformer to aid troubleshooting and remove it when we were finished. Supervisory inertia was the only reason to run ungrounded. Yes you could run with a ground on the system but the time lost troubleshooting other problems wasted more time than was saved by the ungrounded system.
An anecdote from the same plant;
One of the electricians was sent to one of our plants because a ground detector lamp was out on a 440V ungrounded MCC. two or three days later he came into the shop and threw his tools on the bench and announced. Well;
"It's fixed."
The supervisor asked where the ground was.
"No ground. The damm light was burned out."
My first thought was;
"Thank god that wasn't me!"

Bill
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"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

Thanks, gentlemen. My first time in southern Africa and I thought they might do things different here. I showed the young engineers how to use a light bulb to chase the ground.

 

[jraef]

I had a technician working for me that came from the Navy, worked on a Nuke sub. He never grounded CPT secondaries. Drove everyone crazy but the big boss thought it was an interesting quirk and allowed it, he had cut his teeth on IBM mainframe power supplies, he said they floated everything there too.

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