Separate current transformer?

[suebutcher]

Originally Posted by buggies

Agree with the isolation aspect. I think that is what they are trying to describe where the current is separate from the local mains. They specify no earth so that in order to get a shock you would need to contact two conductors in the device under test not just one.

Touching two conductors at once is less likely than one. Without the earthing, if you touched the active you'd just get an unpleasant but non-lethal jolt, yes? But then you'd have no protection from the RCD in the building's mains power box. I'd prefer to have an operating RCD, and refuse to touch live chassis appliances, which in any case aren't likely to turn up for repair here in Australia. But that's just my preference. The other factor is the 240v to 240v high power isolation transformers available here are quite expensive; they're not used much, it's mostly 240v to 110v or 24v for building sites. I've explained the dilemma to the local Repair Café organiser, and suggested she contacts an electrician for advice.

[stuarth]

I’m struggling to understand how an RCD is better than an isolation transformer when working on live chassis sets.

With an RCD, then, if you are not fully insulated from earth and you touch the HT rail (or the chassis if the mains connection were crossed), then you would get an electric shock, and the RCD would trip to limit the duration of that shock.

If you unplug that set from the direct mains connection and plug it into an isolation transformer, now when you touch either HT or chassis you will not feel a thing because the transformer secondary is fully floating with no earth connection for the current to flow through you.

The RCD limits the duration of the shock, the transformer reduces the likelihood of getting a shock.

Of course if you touch HT while also touching the set chassis, you will get a shock which the transformer will not protect you from, but with the direct connection to the mains, neither will the RCD because the current flow would be from live to neutral, the RCD will not trip if live and neutral currents match.

I think the repair cafe advice to use a transformer is sound, but does not guarantee safety.

Stuart

[buggies]

Originally Posted by suebutcher

Touching two conductors at once is less likely than one. Without the earthing, if you touched the active you'd just get an unpleasant but non-lethal jolt, yes?

Neither wire of the isolation transformer secondary is "active" with respect to earth (or you body) so you could hang on to either conductor with your feet in the bath and not feel anything (apart from wet).
Assuming a decent transformer.

Crossed with stuarth.

[suebutcher]

Originally Posted by stuarth

I’m struggling to understand how an RCD is better than an isolation transformer when working on live chassis sets.
Stuart

I'm not saying that it is; certainly if we were going to be working on live chassis appliances, then I agree isolation would be needed. But we won't be touching live chassis gear. With isolation you lose RCD protection, and an earthed chassis appliance will at least have to have a final test on mains with an earth connected, or how are we to know it's fit for use? We need a ruling from a qualified local electrician about what is acceptable locally; I don't think a nebulously worded safety recommendation from the Netherlands is enough to go on. No offense to you chaps, I can see both side of the argument here, but the decision has to have the stamp of authority.

[suebutcher]

Originally Posted by buggies

Neither wire of the isolation transformer secondary is "active" with respect to earth (or you body) so you could hang on to either conductor with your feet in the bath and not feel anything (apart from wet).
Assuming a decent transformer.

Oo-er, I wonder if I dare test this?!

[Skywave]

Re: post 23 by Stuarth.
Agreed entirely. That is what was in my mind in my post 10. And yes, an isolation transformer cannot "guarantee safety". In life, very few - if any - things can do that. But with regard to electrical safety, many measures can be taken to reduce the risks of receiving an electrical shock - an isolation transformer, properly implemented, being one of them.

Al.

[Skywave]

Re: post 22 by suebutcher

Quote: The other factor is the 240v to 240v high power isolation transformers available here are quite expensive; they're not used much, it's mostly 240v to 110v or 24v for building sites. Unquote.

You can connect two 240v. to 110v. isolation transformers 'back-to-back' to produce a 240v. to 240v. isol. transformer: 240v. to 110v.; 110v. to 240v.

Al.

[stuarth]

The bathtub scenario, imagine you had a 240V battery by the bath. If you were to touch one terminal, and there was no path into the bath water from the other terminal (a damp towel?), then you would be quite safe. For the isolating transformer, capacitance to ground from the transformer secondary replaces the damp towel, but such a return path would (should) be pretty high impedance at 50 Hz.

Once the set has been returned to a non-technical user, a different set of problems occur. You cannot guarantee that the chassis is connected to mains neutral, so all the insulation for external sockets (aerial, gram inputs, speaker outputs where present), knob fixings, back panel fixings and ventilation slots, etc must safe with the chassis connected to mains live, a daunting task in my opinion , especially if the original design were marginal in this respect. For a set with a double wound mains transformer, then fitting a 3-core mains cable and earthing the chassis should give adequate safety.

Stuart

[Skywave]

Preamble.
Earlier on in this thread, I chose not to present my thoughts on the topic of using an isolation transformer for fear of taking this thread off-topic with the likely consequence of the thread being closed. However, others have made subsequent similar posts that may indeed bring about that result. (Which I am sure none of us want.) So, for that reason, I feel I now have licence to present those thoughts of mine.

Two scenarios.
(a) 3 wires from a normal mains plug connected to U.K. mains, (nominal 230-v.). Those 3 wires to 3 terminals: L, N and E (usual abbreviations).
(b) 3 wires from a normal mains plug connected to U.K. mains, (nominal 230-v.). The L and N wires connected to the primary of a 230-v. to 230-v. isolation transformer; the E wire connected to the transformer's frame. The secondary of that transformer, plus a wire from the transformer's frame, to 3 terminals: call them L', N' and E.

Risk analysis.
In (a), 230-v. exists between L & N and L & E: two dangerous possibilities.
In (b), 230-v. exists between two terminals only: L' and N'. (Assuming a decent transformer with a very low pri. to sec'y. leakage current). Only one dangerous possibility: L' and N'.

Conclusion.
On the basis of reducing the chances of receiving an accidental dangerous shock, (b) is an improvement over (a).

Aside.
If the secondary of that transformer has a centre tap and is connected to the transformer frame (and thus to the supply E), the 3 terminals will have 115-v. between L' and E and 115-v. between N' and E, the two 115-v. voltages having a 180° phase difference. So now there are 3 chances of receiving a dangerous shock - but two of those are at 115-v.: a 'less dangerous' voltage - but still enough to hurt and still possibly large enough to kill you. For most situations, this arrangement would not be used: scenario (b) is the more usual.

Al.

[ms660]

In b) make sure the terminals aren't in the form of a normal 3 pin outlet socket.

Lawrence.

[Skywave]

If I run the supply earth plus the two 'hot' wires from the transformer to a typical 3-pin socket, suitably labelled, is there a possible problem or a hazard in doing that? If there is, please tell me, 'cause I can't think of one.

Al.

[suebutcher]

I can't either, but should there be a fuse in the secondary circuit if the appliance doesn't have one? We don't have fused plugs in Australia.

[Terry_VK5TM]

Originally Posted by suebutcher

I can't either, but should there be a fuse in the secondary circuit if the appliance doesn't have one? We don't have fused plugs in Australia.

Short answer yes.

The question then becomes what value should it be, considering the (probably) many different types of appliance that will be plugged into the transformer.

Somewhere in these forums was the comment the primary of an isolating transformer should probably also be fused if it is not already.

[stuarth]

I think b) above has a serious safety issue highlighted in a previous thread by Mark Hennessy.

At the BBC, they use isolating transformers for visiting groups’ guitar amplifiers. They found that, if the amplifier mains plug had its neutral and earth crossed, then you could get dangerous voltages between the earth on that amplifier and true earth.

After reading that, I removed the earth connection to the 3pin socket on my isolation transformer.

Stuart

[Synchrodyne]

Australia/New Zealand Standard 3000:2018 “Wiring Rules” might well be relevant here.

I haven’t seen the new issue, but I have extracted the pertinent part, on separated (isolated) supplies from the previous edition, 3000:2007, as an informative but not a normative reference.

NZS 30002007 Amd 1 pp.304-308 Electrical Separation.pdf

Points to note are:

The isolating transformer must meet AS/NZS 61558. That may well preclude the use of ad hoc solutions, such as back-to-back 230/120 volt transformers.

Whether the transformer frame is earthed depends upon whether it is a Class I or Class II device.

There is double-pole fusing and double-pole switching on the secondary side.


It does look like a case where taking professional advice would be the best option.

Live chassis, that is Class 0 appliances have been illegal in Australia and New Zealand for quite some time – possibly for decades - as have the Class 0I type (basically Class I but requiring a separate earth lead rather than one integrated with the power cable). I am not sure what would be the situation with vintage equipment of either type, though. As Sue has said, live chassis equipment was always quite rare down under anyway. Possibly a public repair facility would want to avoid any liability by excluding Class 0 and 0I appliances altogether. Then the question would become one of the desirability or necessity for a separated supply for repairs on Class I and Class II appliances.


Cheers,

[ms660]

Originally Posted by Skywave

If I run the supply earth plus the two 'hot' wires from the transformer to a typical 3-pin socket, suitably labelled, is there a possible problem or a hazard in doing that? If there is, please tell me, 'cause I can't think of one.

If for whatever reason the mains plug on the DUT has had the neutral wire connected to the earth pin and the earth wire connected to the neutral pin a dangerous situation can occur...

...Suppose that the DUT is plugged in and switched on and it has exposed metal parts that are connected to the earth wire in it's mains lead, because of the earth/neutral reversal the exposed metal parts will be connected to one end of the isolating transformers secondary winding, the other end of the secondary winding will be connected to mains protective earth via the DUT's transformer, that means that those exposed metal parts are now at a potential (reference to mains earth) that's equal to the voltage across the isolating transformers secondary winding (240 volts say) Not good...

You could argue that...."Well I always check the mains plug first or whatever" However that particular risk is there and it's a risk that is easy and cheap to to eliminate.

Lawrence.

[Skywave]

That dangerous situation of a mis-wired plug was something I hadn't considered when I wrote my earlier post.
And that is probably because I have never met such a scenario. However, when a 'stranger' does land on my bench, (which is not often) I always examine the wiring in the plug (if it has a removable top, otherwise an Ohms test is made) and check what fuse is fitted. But I can now see the logic of your earlier suggestion. So thank you for your enlightenment.

Al.

[Ambientnoise]

Many years ago, I dealt with mains powered equipment where things such as hydraulic pumps or lighting was deployed in the sea. The then normal scenario was a surface isolating transformer with a centre tapped secondary. Earth isolation was continually monitored by looking at the centre tap leakage using dc from a high impedance source. In addition, an rcd was installed on the secondary side. If either device triggered, the supply was isolated but also a crowbar system shorted all conductors to dissipate stored energy, say in a rotating motor, to further limit risk. Interesting systems, I am sure things have moved on.

[dominicbeesley]

Originally Posted by ms660

You could argue that...."Well I always check the mains plug first or whatever" However that particular risk is there and it's a risk that is easy and cheap to to eliminate.

I should! Last year I unearthed an old live chassis radio, plugged it in and it tripped the RCD...reset, it happened again. Now why would a live chassis set like that trip? I opened the plug to find that some "idiot" had shorted the earth pin to neutral...then I remembered, I did it - an utterly stupid bodge when I'd misplaced the earth clip for my scope probe. I'd had to put things away in a hurry and forgotten to remove the link. Very stupid I know but I often do stupid, forgetful things. In hindsight (I did this when I first started out on repairing vintage equipment) I wouldn't do anything that daft now, honest. [Says the man who forgot to put the lid on the thinners and left it next to the welding bench last week...]

I always use an isolating transformer now - it provides some safety but of course that is never a given, other precautions need to be taken. I try and remember to treat the set as if was still mains live. (The worst culprit is the Decca Bradford - I always manage to get a tickle from the heatsinks of the CRT drivers when doing the convergence)

I'm not a fan of RCD's I'd prefer them to be the last resort - I've had a 30mA belt, up a ladder, and I didn't like it one bit. I also have them in my damp rented workshop the lights going off at 11pm easily leads to a very dangerous situation when you're lying under two tonnes of rust.

The main reason, for me to use an isolating transformer, is like the OP has found - it's all to easy to short the chassis while the back is off: tripping the RCD and inviting the attention of the Health and Safety officer when Strictly goes off...

D

[suebutcher]

Originally Posted by Synchrodyne

Australia/New Zealand Standard 3000:2018 “Wiring Rules” might well be relevant here.

Thanks, that makes the local requirements clearer.