11-02-2015, 10:59 AM
OK, what Al initially described in post #3 was, as he rightly stated, a device to limit the maximum potential w.r.t. mains earth to 120V, thus limiting the magnitude of an electric shock. Of course, similar devices are widely used in the construction trade, for example, where the transformers are 110V with a centre-tap that is connected to mains earth. Hence, maximum potential to mains earth is only 55V, and that brings about a useful increase in site safety - especially a site that might be exposed to the great British weather 
However, such devices do not provide the facility to safely connect earthed test equipment to them.
If the centre-tap is earthed, then a downstream RCD can be connected, and does provide a degree of protection.
But, it's important to be clear that just because a transformer is at the heart of this setup, it does not provide isolated mains. For anyone who is considering replicating this, my personal view is that it doesn't actually "bring anything to the party", over and above regular mains outlets that are protected by an RCD in working condition. Apart from perhaps reducing the magnitude of a shock received during the 30ms that it will take the RCD to open... And I suppose that a large, heavy isolation transformer will offer some protection against spikes on the incoming mains.
But...
Al then went on to say (in post #5) that actually, the earth connection of the outlet sockets is not in fact connected to mains earth any more, but continues to be connected to the centre-tap of the isolating transformer. He goes on to say that the maximum potential between mains earth and his "live" and "neutral" terminals can only be 120V, but this is not guaranteed to always be the case:
With the output from the transformer now floating, a connection between the post-TX "neutral" and mains earth will result in his post-TX "live" becoming genuinely live with respect to mains earth. The same would apply to a properly implemented isolation transformer, of course. But here's the rub: the "earth" connections in his post-TX sockets are now at 120V with respect to mains earth, meaning you could get a shock from the metal case of a class 1 instrument!
If you want to work this way, you need to have a totally earth-free environment - nothing at all connected to main earth, and you need to be unable to connect yourself to mains earth (good rubber matting on the floor, etc). It's a complicated way to work, and doesn't bring any obvious advantage over the much simpler way I described earlier.
In other words: Al, please consider revising this setup. Anyone else: please do not replicate this setup
The subject of isolation transformers is a complex one, and it's one that seems to confuse many a fine engineer and technician. In my experience of discussing this subject on the various vintage forums, I find that many people seem to temporarily forget how electricity works, and instead fall back on "it's how we used to do it at {insert name of former employer, the bigger and more well-known the better}, and we never had a problem, mate!". Well OK, but I always try to encourage people to go back to first principles, and consider all of the scenarios, so at least they can fully understand the benefits and limitations of their chosen setup.
And yes, in any thread about the use of isolation transformers, the subject of electrical safety is very much on-topic. Fairly obviously, I would have thought
For what it's worth, I have 3 types of sockets on my bench:
Hope this helps to clarify a few things,
Mark
However, such devices do not provide the facility to safely connect earthed test equipment to them.
If the centre-tap is earthed, then a downstream RCD can be connected, and does provide a degree of protection.
But, it's important to be clear that just because a transformer is at the heart of this setup, it does not provide isolated mains. For anyone who is considering replicating this, my personal view is that it doesn't actually "bring anything to the party", over and above regular mains outlets that are protected by an RCD in working condition. Apart from perhaps reducing the magnitude of a shock received during the 30ms that it will take the RCD to open... And I suppose that a large, heavy isolation transformer will offer some protection against spikes on the incoming mains.
But...
Al then went on to say (in post #5) that actually, the earth connection of the outlet sockets is not in fact connected to mains earth any more, but continues to be connected to the centre-tap of the isolating transformer. He goes on to say that the maximum potential between mains earth and his "live" and "neutral" terminals can only be 120V, but this is not guaranteed to always be the case:
With the output from the transformer now floating, a connection between the post-TX "neutral" and mains earth will result in his post-TX "live" becoming genuinely live with respect to mains earth. The same would apply to a properly implemented isolation transformer, of course. But here's the rub: the "earth" connections in his post-TX sockets are now at 120V with respect to mains earth, meaning you could get a shock from the metal case of a class 1 instrument!
If you want to work this way, you need to have a totally earth-free environment - nothing at all connected to main earth, and you need to be unable to connect yourself to mains earth (good rubber matting on the floor, etc). It's a complicated way to work, and doesn't bring any obvious advantage over the much simpler way I described earlier.
In other words: Al, please consider revising this setup. Anyone else: please do not replicate this setup
The subject of isolation transformers is a complex one, and it's one that seems to confuse many a fine engineer and technician. In my experience of discussing this subject on the various vintage forums, I find that many people seem to temporarily forget how electricity works, and instead fall back on "it's how we used to do it at {insert name of former employer, the bigger and more well-known the better}, and we never had a problem, mate!". Well OK, but I always try to encourage people to go back to first principles, and consider all of the scenarios, so at least they can fully understand the benefits and limitations of their chosen setup.
And yes, in any thread about the use of isolation transformers, the subject of electrical safety is very much on-topic. Fairly obviously, I would have thought
For what it's worth, I have 3 types of sockets on my bench:
- The first are red in colour, and these have a double-pole isolation switch (with a red rocker). These are left on 24/7, and they are for the computers and anything else that needs to be continuously powered.
- The second type are white, and these also have a double-pole isolation switch and an RCD. These power all test equipment and DUT (the latter via a 4 way mains distribution unit that has voltage and current monitoring built in. Very handy).
- The final type are isolated. I have a pair of 500VA mains transformers mounted in a 3U rack case (details here), supplying single sockets with no mains earth connection. Obviously, no RCDs involved here.
Hope this helps to clarify a few things,
Mark
