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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 11th Aug 2004, 6:40 pm   #1
Radio_Dave
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Default Isolating transformer questions

Hi,

I keep wondering whether to buy an isolating transformer for safety reasons. However I dont really understand how this item provides protection. I assume it's only needed for ac/dc sets?

Everybody always says use an isolating transformer but nobody ever says why! Can somebody please explain

David
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Old 11th Aug 2004, 7:10 pm   #2
paulsherwin
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Default Re: Isolating transformer questions

An isolating transformer allows you to earth the chassis of the set being worked on, even if it's a transformerless set. This is inherently safer (no chance of a live chassis by mistake) and allows the use of earthed test gear like signal generators and scopes.

Opinions differ as to how important an isolating transformer is. It all depends on the environment you work in and whether you use test gear a lot. Needless to say, if you *don't* use a transformer you need to be very, very careful about mains polarity.

HTH, Paul
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Old 11th Aug 2004, 7:21 pm   #3
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Default Re: Isolating transformer questions

HiPaul,

Earthing the chasis makes sense but what, if any, protection would I have from electrocution?

Thanks

David
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Old 11th Aug 2004, 7:25 pm   #4
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Default Re: Isolating transformer questions

If you have a AC/DC set, then the chassis is connected to one side of the mains. If it's the live side then the whole chassis is live. I'm sure that most forum members have been caught somewhere and lived to tell the story. My initiation was on a Bush TV80 " portable " TV when I was about 15 years old. I knew to test the mains polarity with a neon screwdriver but somebody had bridged out the neutral side of the mains switch. I turned off the set and touched the chsssis. Only once.

Actually I think all kids ought to get an electric shock at some point fairly early on just to prove to them that electricity is dangerous if you do the wrong thing. The only problem is that we might kill a few by mistake. It's called un-natural selection!
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Old 11th Aug 2004, 7:48 pm   #5
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Default Re: Isolating transformer questions

Quote:
Earthing the chasis makes sense but what, if any, protection would I have from electrocution?
The main safety benefit is to ensure that the chassis is never live. This is particularly important when working in a proper workshop environment where you'll be surrounded by earthed equipment and metal fittings. In a typical 'dining table' workshop, most of the stuff around you will be insulating or floating so the live chassis is unlikely to give you a serious shock. (Most of us will have touched a 240V live connection at some time, and it's pretty unpleasant, but it's not dangerous if you're not earthed anywhere).

Even with an isolating transformer, if you grab hold of mains live with one hand and the chassis with the other, there's a good chance you'll die. An old repairman's trick was to work with one hand in your pocket.

Best regards, Paul

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Old 11th Aug 2004, 7:55 pm   #6
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Default Re: Isolating transformer questions

Thanks Paul you've answered all I needed to know

David
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Old 11th Aug 2004, 8:02 pm   #7
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Default Re: Isolating transformer questions

Hi All

Quote:
An old repairman's trick was to work with one hand in your pocket.
I would amend this to one hand behind your back. A friend of mine was working on an amateur transmitter with one hand in his (trouser) pocket. He clipped the o/p valve anode top cap (approx 600 v), the reflex caused him to jerk backwards against a door frame. As he couldn't get his arm free in time, he hit it with a lot of force, the outcome was he broke the arm .

Yes high voltages can be dangerous in more ways than one.
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Old 11th Aug 2004, 9:20 pm   #8
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Default Re: Isolating transformer questions

The main advantage of an isolating transformer is as Paul said, the ability to used earthed test equipment. Also you don't need to worry about what way round the chassis is connected on AC/DC sets (or those run from autotransformers or 'heater transformers' like some 'midget'sets are).

In isolation transformer will NOT prevent electrocution but reduces the risks.

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Old 12th Aug 2004, 9:36 am   #9
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Default Re: Isolating transformer questions

I would say that the only advantages of an isolating transformer are for use with earthed test equipment, that it might limit the maximum short current in the event of a problem and that it isolates you from mains neutral - which can become momentarily dangerous when some (other) appliance decides to blow a fuse. If you joined earthed test equipment to mains neutral you might find a lot of current flowing between them and a house RCD would probably trip.

There are disadvantages.
It may lull you into a false sense of security and it prevents any RCDs that might be on the mains side from helping you.

The point is that if you allow one side of the isolated output to be joined to mains earth then the live terminal has EXACTLY the same lethality as the original mains did.
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Old 1st Sep 2004, 11:19 am   #10
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Default Re: Isolating transformer questions

Hi All,

Being new to your forum, I don't know if it is appropriate to ask a question here, or whether I supposed to start a new thread, but I MUST CLARIFY the following:

I thought I understood the correct use of isolating transformers until I read this here:

Quote:
There are disadvantages.
It may lull you into a false sense of security and it prevents any RCDs that might be on the mains side from helping you.

The point is that if you allow one side of the isolated output to be joined to mains earth then the live terminal has EXACTLY the same lethality as the original mains did.
Does this mean that there would be circuit where the current flow caused by a short could not be sensed by the RCD and the person could be electricuted?

Regards
Andrew

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Old 1st Sep 2004, 12:08 pm   #11
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Default Re: Isolating transformer questions

RCDs do not detect short circuits or overloads. They sense the difference betweenthe currents flowing in the live and neutral wires. Any difference is assumed to be a fault, possibly current flowing through your body.

Any fault on the output side of an isolating transformer cannot unbalance the live and neutral currents at the primary so an RCD in the primary circuit cannot give any protection.

An RCD in the secondary circuit can give no protection unless one side of the secondary is earthed before the RCD. THis is a rather odd way to use an isolating transformer and rather defeats the purpose.

The HT on an AC set with a mains transformer is always potentially dangerous. An RCD will not save you as the HT is on the secondary of an isolating transformer. In this case the transformer is in the set, not external, but the idea is the same.

THe main purpose of an isolating transformer is to ensure that the chassis on an AC/DC set is not at mains live.

There is no substitute for being aware of the risks and taking due care.
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