Control Transformer Grounding in Washdown areas

[Handasee]

By code in Canada, control transformers must have there secondary side grounded. I have come across a number of installations in the food industry where the secondary side of the control transformer is not grounded. This is only on machinery that has large amount of water used in the process, and the machinery is washed down daily. The manufacturer said this minimizes the chance of an electric shock to the operators.
Is there any validity to the manufacturers arguement for not grounding?

 

[busbar]

The manufacturer’s logic is flawed. Ungrounded-secondary circuits can float above “0” (ground) or “120“ volts that is predictably found in machine-tool control circuits. Transient ground faults on ungrounded circuits may raise voltage to ground well above the nominal RMS value. Primary-to-secondary faults or leakage in the serving control-power transformer may also cause overvoltage conditions. Both conditions will increase the likelihood of control-circuit insulation failure, and likely increase electric-shock hazard.

 

[PwrCntEE]

The purpose for the ground connection is to create a path of least resistance to protect anyone who comes in contact during a failure. Not installing the ground will, without question, increase risk to personnel. Not to mention it is an obvious code violaiton to not ground.

 

[jbartos]

Suggestion: The grounded secondary of control transformer is used for fast circuit interruption (blown fuse) in case of a short. In very rare cases of arcing, the voltage may be impressed on conductive metallic parts before the fuse clears the circuit. This may cause potential hazard.

If the control transformer secondary is not grounded, a fault can impress voltage on metallic parts without tripping a circuit breaker or blowing a fuse. This voltage is hazardous with respect the other end of winding and that part of circuitry since the full secondary voltage is faced.

However, the control transformer secondary grounding should follow industry standards and electrical safety code including Authority Having Jurisdiction.

 

[Interrupter]

We typically do not ground the control transformer secondary. Expectation is that a ground fault does not lead to stoppage of a motor. We, however, need to make sure that the secondary control wiring insulation resistance is high enough. During maintenance period, we check the insulation resistance of the control circuit line-to-ground and line-to-line. I myself would like to have the secondary circuit grounded so that degraded insulation resistance can be detected by blown fuse. Note that the fuse may not blow if the resistance to the ground is high upon fault. I do not your code requirement,but would it be possible to have lower control voltage on your control circuit? That would reduce risk to your operators even if a fault voltage is applied to your metallic enclosure of your control circuit.

 

[PwrCntEE]

Interrupter:

I hope I am misunderstanding you. Do you mean that you do not ground the transformer so that if you do have a fault, you size the insulation rating to handle the fault and not arc?

If so, this is only acceptable if you have a Impendance Grounding System installed. Otherwise, regardless of the insulation level, you are leaving the fault current to find its own path to ground. Which can be thru personnel.

 

[jbartos]

Suggestion: In some applications, the transformer secondary sides are not grounded, e.g. isolation transformers.
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http://www.elevatordrives.com/elevsupportapptipdecember2002.htm

http://www.quadtechinc.com/newsletter/035125.pdf

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