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Safety in situation where Isolation transformer is misused

nigelpye

Nov 26, 2021
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I am a sound and lighting engineer hiring out equipment to schools and clubs for small events (speech days, am dram etc.) I have recently picked up a client that is hiring LED and incandescent lights, amplifiers, and the necessary control equipment. The client runs 6 separate teams doing pantomime shows for schools during November and December and each show has two lighting tripods with two LEDs and one 750W incandescent on each. The operator has an amplifier, sound and lighting desks and radio microphones. Some of this equipment is full mains, others have PSU's of various outputs from 9 to 15V.

My worry is that the client is absolutely insistent that the equipment is isolated from the venue's supply using a single 1.5kVa Isolation Transformer (IT) and will not budge from this attitude.

I know very little about the workings of an IT but have gleaned that the principle use of an isolation transformer is purely to allow the connection of test equipment to a device under test. I also see that if there are multiple devices plugged into the isolation transformer, then those devices are no longer floating with respect to mains earth. Every part of them will either be at earth potential or some other potential with respect to mains earth. That means that user is at risk of an electric shock - perhaps just from touching the case of another item of gear.

I have also found that the IT does not supply a ground link between the input and output Earth pin, therefore making an RCD inoperative and putting the electrical equipment’s casings and any conductive equipment attached to the circuit at risk of being raised to mains potential without anyone being aware should a fault occur within one or more items.

My feeling is that this is an incorrect use of an IT and the client is putting her teams at risk of a lethal shock should any item go to a fault condition. As I am the hiring Company, I am responsible for the safety of the users and I cannot avoid liability, even though it is the client’s insistence to use an IT.

Please can someone explain whether I have understood the reason for using an IT and that I really should insist on replacing them with a rapid action RCD.
Thanks

Nigel Pye
 
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Minder

Apr 24, 2015
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IMO any equipment that operates above the considered safe voltage of 60v should be earthed grounded, and any voltage above this, should be referenced to a qualifying Earth GND conductor.
 

Bluejets

Oct 5, 2014
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Client is probably trying to cover his butt against any comeback should any of his gear fail.
Isolation transformers in the past were in common use in households (usually laundry) and still used in operating theatres.
These days earth leakage are the common safety link.(some countries called safety switches)
Either way, anyone hiring out or using mains equipment on a commercial basis should have all equipment tested by and authorised electrician and tagged as being safe.
Tags usually have the test done date and the next test due date as well as the testing electrician authorisation number.
These may vary from country to country.
However, I'd be making certain anything from anywhere plugged in to your mains is protected downstream by an earth leakage device permanently installed on the switchboard by a licenced electrician.
These units also require periodic testing and recording of trip times and level of trip current in commercial premesis or schools etc. etc.
If your equipment, make certain a portable earth leakage unit goes with each device.
 
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Martaine2005

May 12, 2015
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There is no reason for him requiring an isolated supply.
Other than having a bad audio setup that constantly gives him trouble with ground loop hum etc.

Have you asked him why?
He may not know and is just copying a sound engineer from a studio environment.

Martin
 

nigelpye

Nov 26, 2021
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There is no reason for him requiring an isolated supply.
Other than having a bad audio setup that constantly gives him trouble with ground loop hum etc.


Have you asked him why?
He may not know and is just copying a sound engineer from a studio environment.

Martin
Thanks Martin.
The client is trying to isolate the lighting and sound kit power from the school or club supply. He has been told that the best way is to use a IT so that's what he insisted on, he has supplied them while I supply the kit (all PAT tested and in date). However this setup denies the use of a RCD and as far as I am aware it allows all the units to reach mains potential via the linked earth if a positive lead touches the casing in a failed unit.
Can the IT be bypassed safely by a separate earth link to the venue's supply?
 

Martaine2005

May 12, 2015
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Can the IT be bypassed safely by a separate earth link to the venue's supply?
Yes, but then it won’t be an isolation transformer! And completely redundant. Merely a 1:1 extension with another transformer in the middle.
If the IT has two or more outlets, the normal approach would be to link the earth terminals (outlets only). This Would blow the IT fuse if two devices have different faults. Namely, neutral to chassis on one device and live to chassis on the other. I’m sure I’ll be corrected if I’m wrong.
Regarding ground loops, afaik, are mainly caused by using different outlets in a house/venue where the earth/ground are at different potential.
There is still no reason for the client to require an IT.
In fact, he could be breaking local laws by NOT grounding a device that requires ground.
Don’t accept “I require”. Ask for the technical reasons.

Martin
 

davenn

Moderator
Sep 5, 2009
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My worry is that the client is absolutely insistent that the equipment is isolated from the venue's supply using a single 1.5kVa Isolation Transformer (IT) and will not budge from this attitude.

And here is the response

Don’t accept “I require”. Ask for the technical reasons.

get the reason or dont hire, since it puts you in a position of liability

your response to her is pretty clear

cheers
Dave
 
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