Earthing of control transformers

[Andiri]

In the designing of an extension to our plant, the issue of operators experiencing electrical shock from operating the push buttons on numerous existing IP65 rated field stop/start stations has raised a few possibly thought provoking questions. The debate is about which control voltage to use with the installation of a "simicode" controller/relay - 24VDC or 110VDC. The 24V option requires extra relays (apparently local standards) vs the existing 110V option which is currently a shocking experience. Here are 2 questions I was asked to raise on the forum.
1) Why do we earth a trf?
2) Would disconnecting the earth from the control trf help prevent electrical shock being experienced at the stop/start stations, since you would be breaking the return path of the leakage current fed from that trf?

 

[bigamp]

Operators getting shocks from operating field pushbutton stations is indicative that something is very badly wrong with the whole installation and this should be addressed immediately.

I do not understand the association between transformers (AC) and your stated control voltages (DC). DC is normally from battery banks which should be referenced to earth in some way. Could you please clarify.

The stop-start station enclosures themselves should be earthed (as should any stands they may be mounted on) and if AC control voltage is used that voltage must also be referenced to earth. That way, if you get an earth fault on a field stop-start station you get a circuit and the control fuse blows.

 

[Andiri]

Bigamp
Oops! That is supposed to be (AC).
The stations (5"X3&quot used are made of a non conductive material, but have an earth connected to the earth connection point inside the enclosure. Could the operator be completing a circuit for the leakage current from the station through the structure of the plant back to the trf?

 

[pablo02]

Adar:

As bigamp stated, it does appear that something is not right with the installation. The problem will not be resolved safely by disconnecting the transformers from earthing...

there are several reasons for grounding (earthing) the transformer -- foremost is safety -- however, failure to maintain that ground with solid continuity throughout creates a hazard...

1) have you done a voltage check between the operator's (start/stop) stations and a solid ground? also to the ground connection in the station? check for both AC and DC; check for both 120v as well as volts < 1 volt... check it to the floor..

2) since these are non-conductive (or supposed to be)stations, is there a chance your operators are building up a static charge? I would assume there is no carpet and the floor is typical industrial (no special coatings?) what type of equipment is nearby? anything like capacitor banks that would induce a capacitive charge?

3) check your ground connections -- does it use conduit and / or cable trays for its earthing? are these properly bonded or has time / plant mods made your connections have an impedance and not adequately earthed?

4) has the building structure been properly earthed? (so there is no potential between the floor and the equipment that may be properly earthed?)

5) check out any lighting circuits in the area for any paths to ground -- these sometimes may not have a solid enough ground to operate a breaker, but sufficient to create a step potential under some conditions and a shock. Is all the equipment properly grounded (for the same reasons)...

6) take a hard look at the wiring to the stations themselves -- is the insulation good, are the wires properly terminated, are the switches in good shape, is the ground connection properly bonded to earth?

7) how old is the plant? what type of soil conditions are present? what type of materials / chemicals are used, etc? has the plant ground to earth be checked lately to verify that it is adequately grounded?

8) is the control transformer properly grounded?

9) regardless, the operators are right about bringing this issue up and it needs to be resolved...

sorry about using the terms earthing and grounding intermixed... I am used to the term grounding and not earthing...

 

[bigamp]

Adar,

Thanks for the clarification. I dont know what country you are in, but it may well be a requirement of your electrical code that the control voltage must be referenced to earth so you have no choice. Referencing to earth could be by directly earthing one end of the transformer winding, or if it is 110V AC, have centertapped transformers and earth the center (to limit any voltage to earth to 55V). In any case, I do not think it a good idea to operate the transformers unearthed.

It should not be possible for operators to get a shock from operating a pushbutton, or from touching a control station. If they do, it is suggestive that there is either defective equipment or incorrect wiring and this should be investigated and put right immediately. Can you actually measure voltage from the pushbutton actuator or the control station body itself to earth?

 

[pablo02]

Adar -- bigamp bought up another issue -- you might explain how your control voltage is set up: I am familiar with 110 vac with a hot lead, a neutral (return path) and a separate ground... the ground and neutral are not connected except back at the transformer (and main panel)... is your wiring the same or different? If the (in my example) neutral is connected to the earthing connection also at a different point or in the control station so that the earthing becomes current carrying, then the operators may provide an additional return path and get a shock when making that parallel connection...

 

[busbar]

IMO, ungrounded AC-control circuits will evolve into a huge can of worms from a fault- locating/shock-prevention standpoint, aside from any code restrictions. After normal wear, flexible boots for wet-location NEMA 4X pushbutton operators can still allow electric shock in sensitive users, due to minor water ingress from pinholes. One solution has been to use leaf-type/”flip-lever” operators. An example is page 80 in

http://files.awdm.com/e-files/ra/800/800-sg001b-en-p.pdf.

[24-volt machine-tool control circuits will often assure their own set of operational/troubleshooting headaches.]