Using isolation transformers.

[Argus25]

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=134123

Originally Posted by ben

But I'd like to think we can at least try to steer the vulnerable away from the really risky stuff, just in case.

I certainly agree with this remark 100%.

One of the big risks for electrocution is the hot chassis AC/DC sets both TV and radio. This is because the chassis can be neutral or phase in some cases with reversed plugging. This also causes disasters with attaching test equipment to these units in some cases. So the key, as suggested, is an isolation transformer.That makes a huge safety margin improvement.

The only problem though, that is not often considered, is that the isolation transformer itself defeats the dwelling's fuse-box RCD's, by isolating the RCD.

However it is still safer with the isolation transformer than not for a "hot chassis" appliance. I never run my Bush TV22 set without one.

Also it needs to be remembered that an isolation transformer should only power one appliance at a time.

There is an interesting saying: "the safety and utility value of an isolation transformer is inversely proportional to the number of appliances it powers"

This is because a fault or connection in one appliance can earth one of the transformer's outputs, taking the other output high above ground and creating a shock hazard in the other appliances, which is also non-RCD protected.

[TonyDuell]

Originally Posted by Argus25

Also it needs to be remembered that an isolation transformer should only power one appliance at a time.

There is an interesting saying: "the safety and utility value of an isolation transformer is inversely proportional to the number of appliances it powers"

This is because a fault or connection in one appliance can earth one of the transformer's outputs, taking the other output high above ground and creating a shock hazard in the other appliances, which is also non-RCD protected.

If you have 2 live-chassis sets powered from the same isolating transformer (OK, strictly from the same secondary winding of an isolating transformer) and they happen to be wired opposite ways round, then you end up with the full 240V between the 2 sets of metalwork.

Each on its own would be safe to touch, but touch the pair (probably one with each hand...) and OUCH!

[ms660]

Yes, and don't connect the mains earth through to the earth connection on the transformer's outlet socket.

Lawrence.

[Argus25]

Quite right Lawrence.

There are times of course when working on a set running from the isolating transformer it will get its chassis earthed, say by the earth of a scope probe or signal generator but that is fine for the duration of the test, provided the implication of that is understood.

(I have a scope which has saved my bacon on a number of repairs, the Tek 222ps "Power Scout" made for scoping industrial mains power systems. Both the probe earths & tip are high voltage insulated and totally isolated from ground with special input circuits, so the probe earth and tip can be connected anywhere, across any component in a switch-mode supply or mains system live above ground, without any worries. This makes it a wonderful and safe diagnostic tool. Its very useful to look at the voltage across a component's terminals in any circuit, say a resistor to assess the circuit current waveform, without having to worry about the potentials relative to ground or elsewhere, sometimes I don't know how I ever managed without it).

[ms660]

Yes, a rake in the long grass can also be a neutral-earth reversal in the plug (it happens) metal case or chassis rises up to the transformers secondary voltage, body contact between the case/chassis and anything else that's earth could be nasty.

Lawrence.

[Skywave]

Originally Posted by ms660

Yes, and don't connect the mains earth through to the earth connection on the transformer's outlet socket.

Why? By connecting the mains supply earth through to the mains outlet on an isolation transformer, you then know that all 'earthed' metalwork that you might come into contact with is properly earthed and that you have a balanced 230 v.a.c. supply. Fit an RCBO in the isol. transformer secondary as a safeguard for leaks to earth - add fuses as well for a belt-and-braces approach.

So, have I over-looked something in my line of thinking?

Al.

[ms660]

Looking at the posting time difference Al, your post might have crossed, see post#81.

Lawrence.

[Skywave]

I have, Lawrence. But if a RCBO is fitted in the transformer secondary, won't that trip if a load is connected with the N and E wiring in the plug reversed?

Al.

[ms660]

Originally Posted by Skywave

I have, Lawrence. But if a RCBO is fitted in the transformer secondary, won't that trip if a load is connected with the N and E wiring in the plug reversed?

Al.

For leakage where's the current imbalance? (scroll down to diagram):

http://switchdiagram.blogspot.co.uk/...ansformer.html

Lawrence.

[Hunts smoothing bomb]

I see, the isolated supply effectively becomes non isolated due to one side of the secondary being earthed through the load because N & E are reversed in the appliance plug.

I never spotted this before, I will be making some changes tonight to the isolated supply socket outlet in my workshop!

Cheers

[Skywave]

Just a quick re-cap: the scenario. A 3-pin plug with a load attached, but that 3-pin plug has been incorrectly wired whereby the N and E wires have been swapped over in that plug.

The current that flows out of one terminal of the secondary and through the RCBO and thus to the load does not return, via the RCBO, to the other terminal of the transformer. Hence a current imbalance: RCBO trips.

An isol. transformer, with RCBO on its secondary, is what I use in my workshop to feed the general bench. Just to satisfy myself, I've just performed a couple of tests using a suitably rated power resistor of 2300 Ohms. With that R connected between either the L' and E or the N' and E, the RCBO trips. The notation N' and L' simply refer to the feed from the RCBO to the bench socket.

Al.

[ms660]

Originally Posted by Skywave

Just a quick re-cap: the scenario. A 3-pin plug with a load attached, but that 3-pin plug has been incorrectly wired whereby the N and E wires have been swapped over in that plug.

The current that flows out of one terminal of the secondary and through the RCBO and thus to the load does not return, via the RCBO, to the other terminal of the transformer. Hence a current imbalance: RCBO trips.

An isol. transformer, with RCBO on its secondary, is what I use in my workshop to feed the general bench. Just to satisfy myself, I've just performed a couple of tests using a suitably rated power resistor of 2300 Ohms. With that R connected between either the L' and E or the N' and E, the RCBO trips. The notation N' and L' simply refer to the feed from the RCBO to the bench socket.

Al.

With the diagram posted in the link earlier what is the current path?

Lawrence.

[Skywave]

As shown, the current path is not where it should be!

But that just highlights my point: when using a mains isol. transformer, feed the load on the secondary via an RCBO. Then common all the earth connections.

Al.

[ms660]

Originally Posted by Skywave

As shown, the current path is not where it should be!

But that just highlights my point: when using a mains isol. transformer, feed the load on the secondary via an RCBO. Then common all the earth connections.

Al.

Given the scenario shown in the diagram in the link I can't see what use an RCBO would be unless I'm missing something?

Lawrence.

[Hunts smoothing bomb]

If you take a look at the diagram then you will see that the current does indeed return to the isolation transformer secondary, if the RCBO was connected to this secondary then I cannot see how an imbalance would occur so it would not trip?

Al,

With reference to your test, I do not understand why the RCBO tripped?

If the RCBO is connected to the transformer secondary output and the transformer secondary is isolated from earth then how does the current get via one side of the secondary through the RCBO, to earth and back to the other side of the secondary without passing through the RCBO in order to make it trip?

Cheers

[Skywave]

The return current from the load does not get back to the RCBO and thus to the secondary of the transformer though, does it? It tries to go down the supply earth wire. So the RCBO senses a current imbalance - and trips: that's the whole point of an RCBO, no?

Perhaps I'm missing something obvious here: won't be the first time!

Al.

[ms660]

Originally Posted by Skywave

The return current from the load does not get back to the RCBO and thus to the secondary of the transformer though, does it? It tries to go down the supply earth wire. So the RCBO senses a current imbalance - and trips: that's the whole point of an RCBO, no?

Perhaps I'm missing something obvious here: won't be the first time!

Al.

The current through the load does get back to the transformer, via the bloke in the picture and he doesn't look too happy.

Lawrence.

[Herald1360]

Earth one end of the ISO TX secondary. This is now your Supply Neutral, the other end is Live.

Take the L&N outputs of the ISO TX via the RCD to your set. The set is now, in isolation, protected by the RCD whichever way round it's connected since if you touch one side it's at earth anyway, so no shock and if you touch the other, the return path is via earth direct to the TX bypassing the RCD which trips. You'll still get a belt, but hopefully one you'll remember!

You're no better off than with an RCD on the mains though and still have the same problem with grounded test gear as with RCD protected straight mains.

On balance, isolating the set completely is probably safer, and you can connect your earthed test gear to it. At that point, though, you CAN get belted by the HT to Earth so keep one hand in pocket and stand on the rubber mat, same as for an isolated set.

If you get two parts of your body across the HT in any of these scenarios, there's no protection possible anyway whatever the setup other than luck and your skin resistance!

[G6Tanuki]

A hint: if you're working on AC/DC radios and using an isolation transformer, for additional safety use the 110-120V seting on the transformer and set the radio similarly.

For a couple of years I've been using an Eddystone 840A as my 'bedside radio' for late-night listening - this is an AC/DC model and feeding it from a step-down transformer makes it both safer and also because we're no longer dumping tens of Watts of unnecessary waste-heat inside the case from the dropper-resistor, it runs cooler and so has less frequency-drift whrn listening on the upper shortwave bands.

[Skywave]

I don't think that there's much more I can add. In an earlier thread I described an experiment that I carried out here with my isol. transformer + RCBO and the consequent results. Those results assured me of one safely feature in my 'shop that I'm happy with.

So I'll simply conclude that the danger highlighted in post #5 (N and E reversed in the plug) simply suggests that when an unknown piece of kit lands on your bench, first, before doing anything else, dismantle the plug and look at the connections - and the fuse rating: don't assume.

Al.