Isolation transformers, use of.

[Skywave]

Floating supplies are not allowed to feed any appliance or for any general supply (IEE regs) All supplies of electricity must be earthed. That's why an isolating transformer only feeds one piece of apparatus only, which must not be connected to any earth.

Portable tools for outdoor use are feed from a 110v transformer, some people have called this an isolating transformer. However, this is a double-wound with a centre tap to earth, so that a shock from either side will be 55v.

Isn't there an inconsistency in those paragraphs? Perhaps I've misunderstood you.
Comparing the statement in para. 1 (IEE regs.) to the statement in para.2 (portable tools for outdoor use), seems to imply that those transformers used for portable tools are in contradiction to the aforesaid IEE regs., to wit: to earth or not to earth.

[Mark Hennessy]

Floating supplies are not allowed to feed any appliance or for any general supply (IEE regs) All supplies of electricity must be earthed. That's why an isolating transformer only feeds one piece of apparatus only, which must not be connected to any earth.

Portable tools for outdoor use are feed from a 110v transformer, some people have called this an isolating transformer. However, this is a double-wound with a centre tap to earth, so that a shock from either side will be 55v.

Isn't there an inconsistency in those paragraphs? Perhaps I've misunderstood you.
Comparing the statement in para. 1 (IEE regs.) to the statement in para.2 (portable tools for outdoor use), seems to imply that those transformers used for portable tools are in contradiction to the aforesaid IEE regs., to wit: to earth or not to earth.

Honestly, I don't see any inconsistency there. The first paragraph states that supplies shouldn't be floating. The second paragraph states that tools are powered from a supply that has a centre-tap connected to mains earth - i.e. not floating. I made the same point in the opening paragraph of post #10.

[raylec]

The RCD I mentioned as being pointless and offered no protection. I was referring to the secondary output, the floating supply to the workbench test socket.
The RCD would still protect the primary of the transformer and wiring, however its only earth leakage that is protected,as the only earthed object would be the transformer frame and core, this would normally be earthed any way, and as you would not come into contact with any of the primary wiring,to trip the RCD due to earth unbalance,I dont see any point in using this device.
You either use an RCD to feed your test socket,or feed from isolating transformer, not both.
I would prefer a transformer,which does give some current limiting,should a fault occur.

[Mark Hennessy]

We've decided to move the posts about electrical installation work into a new thread, as it had developed into a very interesting topic in its own right

http://golbornevintageradio.co.uk/forum/...p?tid=4595

All the best,

Mark

[raylec]

An isolating transformer only feeds one piece of apparatus which must not be connected to any earth.

From an earlier post of mine:

That transformer is internally configured for 120v. - 0v. - 120v. bi-phase output with the 'E' / 0v. centre point connected to supply earth inside the commercially-built isolation transformer unit which has full Government electrical safety  approval.

Note: That 'earth through' arrangement has now been disconnected my me.

Al.

These power tool transformers are 110-0-110 centre tap to earth, THEY ARE NOT ISOLATING TRANSFORMERS! they comply with IEE regs for electricity supplies, since all supplies of electricity must be earthed for safety.

[Terry]

As has been said earlier in this thread, the use of isolation transformers is frequently misunderstood.

At one time I worked on special cable TV receivers based on the ITT VC100 chassis but the tuner unit, which had an external wired 22 button channel selector was earthed, as was the distribution system (isolation was in the IF connection rather than the aerial isolators used on domestic receivers).

Initially we used RS isolating transformers which were free standing on the benches and could be moved around as required but we were required to move out of that building into a permanent workshop in the main building. As our network was expanding, we needed more engineers and more bench space, so this came at an appropriate time!

The Chief Engineer was told that we required isolation transformer fitted to every bench position. This point was duly noted and carpenters and electricians moved in to fit the place out.

I and a colleague paid a visit to see how work was progressing. By the door was a consumer unit with mains switch and metal trunking running down to the floor except that part way down was a large square louvred box ...

'What is that?', I asked. 'Oh that's your isolation transformer!' came the reply.

They had summed up the total demand, based on the transformer rating they been given but couldn't find a suitable transformer anywhere so they'd had one wound specially - which would have fed all the benches in parallel!

After we'd pointed out the error of their ways, all the bench positions bar one had individual isolation transformers fitted beneath them.

The remaining bench had a very hefty isolated supply!

[raylec]

The use of isolating transformer to feed workbench,if the bench has multiple outlets THIS IS FORBIDDEN.
All supplies of electricity must be earthed IEE regs.
An isolation transformer is to feed the equipment under test/repair, and not connected to any other device or socket.
Any floating supply can  have dangerous voltages on the poles,due to a number or reasons,hence the requirement to earth one of the poles.
Therefore the max voltage to earth would not be greater than the supply.
If an isolating transformer is used in the proper manner, then it will increase safety when working on the appliance under repair.
Another reason is that the transformer will limit any fault current,and in my opinion,this is the most valid reason for the tranny.

[Mark Hennessy]

Nice to see an old thread pop up again

Having just re-read it, I think that someone innocently stumbling across this should readily appreciate the facts that several of us have set out here. The BBC document in post #2 is a pretty useful reference as well.

It's true that the OP had an unconventional setup - and one that contained a hidden danger - but as some of us will know, he has since moved house and established a new workshop there. I have no idea if he continues to use the arrangement described within this thread, but at least I and others did our best to explain the issues, and we can only hope that he considered our advice.

Naturally, we have a disclaimer that people should read before acting upon advice found on this forum.

All the best,

Mark

[ppppenguin]

I'll reiterate what others have said: The use of isolating transformers is widely misunderstood. The BBC document referenced in post #2 is an excellent guide.

Ultimately isolators, RCDs etc are not panaceas. There is no substitute for understanding the problems and using your brain to avoid damaging your body.

[Refugee]

The BBC document is a good guide but does not quite cover all of the points we need to know when using an isolating transformer.
The OB unit transformers are not wired to provide isolation at all in the mode we are using them for as they are configured to prevent unwanted tripping of an RCCD in a venue that is providing power and for nothing else and operators will be trained to use the equipment accordingly.
What we need is a fully floating secondary that can float about the chassis of the unit under test regardless of the connections in the plug and the secondary terminals of the isolating transformer must both be considered live while the chassis is connected to a local earth usually the chassis of a test instrument.
Both terminals of an isolating transformer can be live at the same time if the temporary earth is connected to the negative of a bridge rectifier as is found in some later model valve TV sets and virtually all SMPSUs. In this case the secondary of the isolating transformer floats about the negative terminal of the rectifier and will be live at both ends with respect to the chassis of the test instrument and the item under test.
This is no problem if only one unit is connected to the isolation transformer and is exactly the reason for using one in the first place.