120 V Control Circuit Grounding

[mjc1060]

I am working on a CRAH (computer room air handler) for a data center. We have had 11 failures of our CRAH ATS switches. The control power for the ATS is 120V. When checking voltage to the control power inputs we had a 120V. When I spoke with an Eaton (manufacturer of ATS switch). I was directed to check the resistance of the control power terminal. The engineer expects to see a resistance between 110 and 120 ohms. I had a resistance over a meg ohm. The engineer believes the device failed because of a voltage surge. Could an ungrounded secondary of a 120V control circuit be vulnerable to a voltage surge? Would grounding the secondary help this problem?

 

[LarryFine]

Could an ungrounded secondary of a 120V control circuit be vulnerable to a voltage surge? Would grounding the secondary help this problem?

In my opinion, yes. One of the reasons our power systems employ a grounded conductor is to fix the voltages relative to earth.

Without grounding one conductor, you could induce 1,000v to ground on one wire and 1,120v to ground on the other.

This practice is standard and usually required. The "neutral" of your control circuit should be bonded to the source EGC.

 

[ptonsparky]

In my opinion, yes. One of the reasons our power systems employ a grounded conductor is to fix the voltages relative to earth.

Without grounding one conductor, you could induce 1,000v to ground on one wire and 1,120v to ground on the other.

This practice is standard and usually required. The "neutral" of your control circuit should be bonded to the source EGC.

I don’t believe the secondary of a 120 volt two wire control transformer is required to be bonded. I’ve seen enough of them in the past. IDK how that relates to the OPs question though.

 

[paulengr]

I don’t believe the secondary of a 120 volt two wire control transformer is required to be bonded. I’ve seen enough of them in the past. IDK how that relates to the OPs question though.

It’s under 250 V. There is an exception for instrumentation but probably not applicable here. It is usually grounded but maybe not obvious.

The problem with ungrounded systems is transients particularly with relay coils and triac putouts known as inductive kick. The other typical problem is arcing or chattering contacts. Arcing can generate nearly unlimited voltages. The second reason is to keep voltages near Earth potential to avoid shock hazards.

I see ungrounded transformers too but often this is a manufacturer decision based on not knowing the grounding arrangement. Users have to connect X0 or X2 to the system ground or to a new ground system if it is to be separately derived. In switchgear due to ground potential rise sometimes floating grounds are necessary but not HVAC controls. Sounds more like improper installs.