but I ran out of editing time by the time I noticed.for protecting sockets for general and/or outdoor use
Red to black wrote:An RCD will not protect you against a live to Neutral shock, as there will be no imbalance, what current is leaving the live will be returning on the Neutral (in actual fact there may be enough earth leakage through yourself that it will trip, but this is far from guaranteed, and I would not like to try it to see).
Terrykc wrote:Mike, something to bear in mind is that, in the unlikely event of finding yourself in the same situation again - please assure me that you won't! - if you can bring the device you are holding into contact with anything remotely earthy, the RCD will trip.
Mark Hennessy wrote:Wow - this has opened a can of worms. I'm not surprised :=D
It's important to remember that just because Employer A did X back in the day, it doesn't make it right, safe, or recommended for current (sorry!) practice.
There are three rules that are utterly rigid:
1. One DUT per transformer secondary.
2. All Class 1 test equipment to be connected to mains earth - earths cannot be lifted.
3. The mains earth should NOT be carried through to the isolated socket.
I'm not sure how much explanation I should attempt to give, especially as it's already been explained (and questioned) on this thread. I'll attempt to explain fully, but as briefly as possible:
1. You are using an isolation to remove the connection between the DUT and the incoming mains so that you can connect earthed test equipment. As you know, mains Neutral is nominally connected to mains Earth at some point, depending on what arrangements you have (another complex issue!). If the mains is full-wave rectified, the live chassis (negative output of the diode bridge) will be at what is colloquially called "half-mains". When you analyse it, you discover that with respect to Neutral, the negative side of the rectifier travels between around +0.6 and -320V, averaging at around -160V. Clearly, this can't be connected to mains earth via a 'scope probe without a fight!
If you insert an isolation transformer, you have broken the connection between the AC input terminals of the bridge and mains Earth. So now, you can ground any part of the DUT you wish. You'll probably want to ground the negative output of the bridge; if so, the secondary winding of the transformer is now floating up and down instead of the negative side of the bridge.
With all that understood, it should be 100% obvious why we have the "1 DUT per transformer" rule, but in case it isn't, ask yourself what would happen if a second DUT had a different PSU arrangement - e.g. half-wave rectification. In this case, you'd be expecting to ground the chassis so that you could connect a 'scope, but the chassis - which would normally be connected to mains Neutral - is now violently moving up and down relative to mains earth because of the first DUT you connected.
Put simply, an isolating transformer provides a floating output. As soon as test gear is connected to a DUT, the output is no longer floating.
2. It was common practice to lift the earth of test equipment, but this practice must be wholly condemned. Quite apart from the performance issues that might occur, the equipment was built to Class 1 standards, and expects to have the metal case firmly grounded so that in the case of an internal fault, the mains fuse can blow before a user receives a shock (Class 2 equipment doesn't have an earthed case, so has higher standards of internal insulation).
So with the earth lifted and the scope probe firmly clipped to the live earth, the whole of the 'scope is at "half mains" for a PSU with an input bridge. Surely I don't have to explain why this is dangerous? Even if the external controls and casework are all plastic, the knobs might have metal grub screws, and the probes will have BNC plugs on the end. Don't. Just don't.
3. Whether to carry the earth through to the DUT socket is often debated. But it shouldn't be - it's pretty clear-cut.
Suppose the DUT has an Earth-Neutral swap in the mains lead. The device appears to be dead because current can't flow through the DUT. But, the case of the DUT is now connected to one end of the secondary winding, and the other end is connected to mains Earth - in other words the case is now live!
It's BBC practice to provide a 4mm binding post connected to mains earth, so an engineer can choose to make the required earth connection before any test gear is connected - this protects the gear against unexpected accidents
All of this is explained in a BBC EGN - point 3 is explained with diagrams in Appendix 1
http://www.bbc.co.uk/safety/pdf/safety- ... v2-1nm.pdf
I hope that this answers the questions about what should be done. Never mind what was done in the past!
Electrical Safety Guidance Notes (EGNs)
The BBC has previously published a series of electrical safety guidance notes. These are too technical to be of use to the majority of BBC users so, with the exception of the note on Portable Appliance Testing have been withdrawn from this site and are held in on an archive on the BBC Safety Teamsite.
If you have a specific need to consult these then please contact BBC Safety. However, you should note that there is no mechanism to keep these updated or reviewed.
Alistair D wrote:Still no reply from the BBC!
http://www.tortech.com.au/isolation-tra ... cd-testing
Can anyone shed some light on how these transformers are wired differently to a normal isolation transformer?
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