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Old 15th Jan 2020, 5:35 pm   #21
'LIVEWIRE?'
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Default Re: Isolating transformer.

If the 3rg pin on such a transformer should not be connected either to mains earth or the transformer secondary, why do the manufacturers provide an earth pin in the first place? Despite over 50 years working with electronics, am I missing something I ought to know?
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Old 15th Jan 2020, 6:07 pm   #22
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Default Re: Isolating transformer.

Is it possible to have a "one size fits all" isolating transformer? The more power you draw from it, the more the output voltage will drop. A 30VA transformer will produce the rated output voltage when it is supplying 30VA. A 500VA transformer will produce the rated output voltage when it is supplying 500VA. Won't a 500VA transformer have an output voltage which is much too high when you are only drawing 30VA? At least that's what happens to me and I usually end up feeding the transformer from a variac.
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Old 15th Jan 2020, 6:09 pm   #23
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Default Re: Isolating transformer.

I have now disconnected the earth from the transformer secondary, but now as you say the RCD will give absolutely no protection if I get a shock from the transformer output to the RF earth.
My reason for the previous earth connection was to simulate the incoming mains but also isolated from it, so I could use an RF earth with no worries of the 2 earths interfering with each other.

I am not convinced that touching any live mains when you are completely isolated from earth results in no shock. I am I have had shocks, not too serious ones, when I have been standing in rubber soled shoes on a wooden or carpeted floor.
I have always suspected that it is possible for the high voltage to give you a shock as it charged up your body and discharges by capacitance to your surroundings including humid air around you.
But I am not going to test it again now to prove the point as they are not pleasant.

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Old 15th Jan 2020, 6:10 pm   #24
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Default Re: Isolating transformer.

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Originally Posted by bluepilot View Post
Is it possible to have a "one size fits all" isolating transformer? The more power you draw from it, the more the output voltage will drop. A 30VA transformer will produce the rated output voltage when it is supplying 30VA. A 500VA transformer will produce the rated output voltage when it is supplying 500VA. Won't a 500VA transformer have an output voltage which is much too high when you are only drawing 30VA? At least that's what happens to me and I usually end up feeding the transformer from a variac.
No I dont think that is going to happen, it may be few volts drop, but if you put 240v in you should get 240v out. But a 500va transformer will have much better regulation than a 30va one.

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Old 15th Jan 2020, 7:34 pm   #25
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Default Re: Isolating transformer.

Quote:
Originally Posted by Skywave View Post
Quote:
Originally Posted by stuarth View Post
Note that if you have a 3 pin mains socket on the output of an isolating transformer, the earth pin should not be connected to mains earth or the transformer secondary.
And the reason for that is . . . ?

Al.
...given every time this topic comes up. Post #11 has links to a few example threads.



PS: this is another conversation about using an isolation transformer to allow the safe connection of test equipment to a DUT. If you are still using your transformer with a centre-tapped secondary, that's a different use-case to what is under discussion here
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Old 15th Jan 2020, 7:49 pm   #26
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Default Re: Isolating transformer.

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Originally Posted by crackle View Post
I have now disconnected the earth from the transformer secondary, but now as you say the RCD will give absolutely no protection if I get a shock from the transformer output to the RF earth.
My reason for the previous earth connection was to simulate the incoming mains but also isolated from it, so I could use an RF earth with no worries of the 2 earths interfering with each other.
Without seeing a diagram of your setup, I might wrong about this, but generally an RCD downstream of an isolation transformer will never trip, nor will an RCD upstream of the transformer (unless the transformer is faulty). This is true whether the earth is carried through or not.

Of course, as soon as you connect earthed test gear to a DUT, then the RCD might function. But again, that is true whether or not the earth is carried through.

If you want an RCD downstream of an isolation transformer to work normally, you have to connect one side of the secondary to earth, thus making phase, neutral and earth on the secondary side. This gets you back to square 1 if the objective was the usual one (allowing safe connection of earthed test gear to a DUT).

But it's not completely without merit: it does stop any RCDs upstream of the transformer tripping should the downstream RCD trip - in other words it allows you to provide discrimination by removing the leakage currents of the secondary side from the primary. This is commonly done on OB vehicles, for example.

Given that you can only connect one DUT to the output of an isolation transformer (when using it in the normal way to allow safe connection of earthed test gear to a DUT), large isolation transformers are pretty useless on the face of it. But if anyone has one of these, and it's not too noisy and doesn't waste too much energy just sitting there plugged into the mains, then using it to provide discrimination for a workshop is actually not a bad idea. Certainly, on the one occasion a faulty bit of gear tripped an RCD here, it was the one in the consumer unit, not the local one on the bench. Murphy's Law!
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Old 15th Jan 2020, 8:12 pm   #27
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Default Re: Isolating transformer.

There are conflicting posts on the use of isolating transformers and we need any advice here on safety to be correct. It might be a good idea to create a sticky on the subject so here is my understanding but we need consensus that we are giving correct advice so please correct me if I am wrong.

1) The purpose of an isolating transformer is to completely isolate the equipment under repair from the incoming mains. That means there should be no direct electrical connection between the mains supply and the supply to the equipment under repair and that includes earth.
2) Using an isolating transformer, an operator can touch a single point on the equipment under repair, HT or AC, and not get an electrical shock as there is no return path to the ground that the operator is standing on.
3) Using an isolating transformer and touching two points on the equipment under repair, such as HT and chassis, will result in electrocution and could be fatal. In this case the current flow is contained within the equipment under repair and an RCD on the supply will NOT provide protection.
4) Any mains powered test equipment used for the repair must also be powered from the isolated supply if isolation is to be maintained.
5) If the earth lead of a scope probe is connected to the equipment under repair and the scope is powered from the mains, then the single touch protection no longer applies, however, if the supply is on an RCD circuit there will be some limited protection.
6) Connecting the neutral and earth together on the isolated socket is advisable as it ensure the earthed parts of the equipment under repair are not floating. This will also cause the RCD to trip if an earthed scope probe or similar is connected to the earthed parts of the equipment under repair.

It is important to note from point 4) that using an isolating transformer with valve equipment does not protect you from shocks from the dangerous potential differences in the equipment under repair.

Last edited by PJL; 15th Jan 2020 at 8:22 pm.
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Old 15th Jan 2020, 8:31 pm   #28
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Default Re: Isolating transformer.

Respectfully, I'm afraid I can't agree with most of those points.

They are a means of connecting earthed test equipment to a DUT. That is all.

I have never considered them to be a device that reduces the chance of receiving a shock, and I would strongly urge anyone who thinks that way to reconsider. I accept that might mean having to change a view that's been held for many decades. There is much misunderstanding and misinformation out there.

There's a very real risk that people labour under a false sense of security when using an isolation transformer, thus making them intrinsicly less safe than not having one.
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Old 15th Jan 2020, 8:44 pm   #29
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Default Re: Isolating transformer.

Quote:
Originally Posted by mhennessy View Post
Respectfully, I'm afraid I can't agree with most of those points.

They are a means of connecting earthed test equipment to a DUT. That is all.

I have never considered them to be a device that reduces the chance of receiving a shock, and I would strongly urge anyone who thinks that way to reconsider. I accept that might mean having to change a view that's been held for many decades. There is much misunderstanding and misinformation out there.

There's a very real risk that people labour under a false sense of security when using an isolation transformer, thus making them intrinsicly less safe than not having one.
Either way, we need a definitive statement as this thread is very misleading and should probably be deleted. I think my points are accurate but I agree entirely, as in my point 4), that they offer no protection to those messing around with equipment with high voltages in them.
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Old 15th Jan 2020, 8:49 pm   #30
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Default Re: Isolating transformer.

If we deleted this thread, we'd also have to delete a lot of other threads

The best thing anyone, including the OP of this thread, can do is to download and study the BBC paper that I always mention in these threads.

For anyone who missed it earlier, here it is again: https://www.vintage-radio.net/forum/...4&d=1488109603

Really, it's all in there, and it's easy to read and digest
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Old 15th Jan 2020, 10:37 pm   #31
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Default Re: Isolating transformer.

Quote:
Originally Posted by mhennessy View Post

They [isolaton transformers] are a means of connecting earthed test equipment to a DUT. That is all.

I have never considered them to be devices that reduce the chance of receiving a shock, and I would strongly urge anyone who thinks that way to reconsider. I accept that might mean having to change a view that's been held for many decades. There is much misunderstanding and misinformation out there.

There's a very real risk that people labour under a false sense of security when using an isolation transformer, thus making it intrinsically less safe than not having one.
Quite possibly the most salient and important statements of the thread so far.

Alan
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Old 15th Jan 2020, 11:54 pm   #32
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Default Re: Isolating transformer.

Re post 21
My isolated variac originally had the mains earth connected to the earth pin of the output socket. I disconnected it after Mark showed the reasoning in the BBC paper some time ago. Draw it out, it’s an obvious risk (well obvious once someone has shown it!), thanks Mark.

Re post 23
There should be only one earth in your premises. Wiring regulations state clearly that all extraneous conductive parts, ie anything which might be connected to ground, should be securely bonded together. This is to protect against earth faults where the RCD or MCB will not protect you. Your RF earth should be the same as the earth from your mains supply.

You do not get a shock from touching any one point, you get a shock from simultaneously touching two points at different potentials, you need a return path for the current through your body. You do not want your isolation transformer to simulate the incoming mains which is ground referenced, and one side sits at 240V above ground, you want it to be electrically separate, ie isolated from ground. Then you will not get a shock by touching an isolated secondary and earth simultaneously, there is no return path for the current from one side of the secondary through your body to the other side on the secondary.

The momentary shock you get when touching door knobs for example are, as you say due to static charges equalising. Imaging you and the door knob as two capacitors to ground, each charged to different voltages. When connected together, the charge voltage equalises.....


A big advantage of using an isolation transformer is that it allows the connection of test equipment earth to any point in the equipment. Another advantage for many folk here is that it also improves safety, especially for the less experienced, or those working on the kitchen table, when working on live chassis sets with reversible mains leads, where the chassis could be at 240V above a nearby radiator, kettle, or whatever.

Anyway, read the BBC paper, disconnect the transformer output socket earth pin and any earth connections to the secondary, and don’t bother with an RCD on the transformer output, it won’t do anything.

Stuart
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Old 16th Jan 2020, 1:26 am   #33
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Arrow Re: Isolating transformer.

Quote:
Originally Posted by stuarth View Post
Note that if you have a 3 pin mains socket on the output of an isolating transformer, the earth pin should not be connected to mains earth or the transformer secondary.
Quote:
Originally Posted by Skywave View Post
And the reason for that is . . . ?
Quote:
Originally Posted by mhennessy View Post
...given every time this topic comes up.
Although I suspected what stuarth's reply would be, I felt that he should have stated his opinion as to the reason 'why' within that particular post, since this topic is one of serious safety. My post was an invitation for stuarth to do just that.

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Old 16th Jan 2020, 1:45 pm   #34
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Default Re: Isolating transformer.

These discussions seem to going round the options for connecting 3 things; the isolation transformer secondary, mains earth, and the earth pin of the transformer output socket.

My view, and I think that of Mark/The BBC, is that none these 3 things should be connected to each other at the transformer.

So why shouldn’t any of them be connected together?

Obviously connecting any point on the secondary to mains earth defeats the isolation. This means you are restricted as to where you can connect an earth at the DUT, and which side of the secondary you connected to chassis. In many cases you will want to earth a point in the DUT which is not a point on the transformer secondary. If you want an earth connection to the DUT, make the connection at the DUT, eg with a test equipment ground, not at the transformer.

Connecting the output socket earth pin either to mains earth or to a point on the secondary has possibly safety problems under specific conditions, as explained in this thread and many others.

Of course you can test for these conditions every time you plug anything into your transformer, but to get the same level of safety as leaving the earth unconnected would require you and your test gear to be infallible.

Mostly, it doesn’t matter what you do with the connection to the earth pin at the transformer, but, for a for safety, you need to consider what happens when it does matter. A lot of safety improvements are due to looking at past problems, and trying to eliminate or mitigate them in future. For a transformer used with a wide variety of equipment design and faults, I would go for the fail-safe option, leave the earth pin open-circuit at the transformer, and, as Mark says, have a separate earth wire if you wanted an earth at the DUT.

Stuart

Just some additional comments

Best practice in any profession changes with time as some safety or other problem arises or becomes more common with others becoming less common or irrelevant. For example look at the evolution of the part P wiring regs with time, or, on a smaller scale, the BBC isolation transformer change to not carrying the mains earth to the output socket.

We need to be aware of these changes, decide if they apply to us, and if so do we need to do anything about it, particularly those things which make our lives safer. I would think that most of us are not, or are no longer, employed by businesses with in-house training courses and rules which must be obeyed on fear of dismissal, we’re on our own to pick up clues where we can (this forum is excellent for that), and make our own decisions accordingly. I know lots of us get great pleasure from working on stuff which would not meet current safety standards (good heavens, some of us work on TV sets with the back off!), but that should not stop us from changing our working environment where appropriate.

Stuart
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Old 16th Jan 2020, 3:39 pm   #35
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Default Re: Isolating transformer.

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Originally Posted by crackle View Post
I have had shocks, not too serious ones, when I have been standing in rubber soled shoes on a wooden or carpeted floor.
But the return path is not through the soles of your feet -- it's capacitive, with one plate being the whole surface area of your skin. This is part of the reason why small children are better at surviving mains shocks than adults!
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Old 16th Jan 2020, 5:57 pm   #36
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Default Re: Isolating transformer.

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Originally Posted by julie_m View Post
But the return path is not through the soles of your feet -- it's capacitive, with one plate being the whole surface area of your skin. This is part of the reason why small children are better at surviving mains shocks than adults!
This is very misleading.

Yes, if you make a single point of contact with the mains then current does flow in spite of there not being a circuit. It's enough to light a neon screwdriver but not enough to kill you, or even shock you. But you can feel it!

I calculate about 8µA based on the standard human body model of 100pF self capacitance.

So if you get an actual shock then there are two points of contact - somewhere!
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