Alistair D wrote:I get the logic of Appendix 1. I have to say though that in 50 years of opening plugs I have never seen one with the earth and neutral leads swapped.
Alistair D wrote:One drawback of the BBC system is that not all equipment has somewhere to securely connect an earth lead to. A croc clip attached to the body of a BNC socket does not qualify as secure.
Well, remembering that we're talking about bench maintenance, where the DUT will have covers removed, and will be in the care of a qualified engineer, so this is not an issue because suitable internal earthing points will be available. But when floating a DUT, it's done solely for the purposes of connecting test equipment
, which will usually be earthed - the safety earth aspect is quite separate - indeed, once connected via the transformer, no current will flow into the earth anyway. In this context, calling them "safety isolating transformers" is a bit of a misnomer.
BTW, a non-isolating bench outlet with RCD protection is the default for bench maintenance; isolation transformers are only ever used in the very small number of cases when you do need to connect test gear to a live "earth" - such as the live side of a SMPSU. This is often at odds with people in the TV repair trade, who default to isolation transformers by default, and often make the mistake of connecting more than one DUT to the same transformer, or who even have the test gear on the same transformer! As many previous conversations on many different forums has amply demonstrated, even the best engineers seem to forget first principles when it comes to isolation transformers!
The other BBC use is performing artists, who bring in amplifiers, etc, in the most shocking states of repair. In theory, a PAT test should be done, but that would bring a halt to many/most performances. Even just taking the cover off the plug top to check the fuse isn't wrapped in foil gets into risky/litigious territory. So an isolation transformer with a circuit breaker and no earth connection carried through removes all risk while they are on BBC premises.
Outside of the workshop, other "isolation" transformers are available for other uses, such as site transformers (55-0-55). Someone I know uses (used, hopefully!) one that is 115-0-115, and it was obviously designed to reduce the magnitude of a shock. However, he'd lifted the earth, but maintained the secondary's centre-tap through to the earth pins of the outlets - which offered some "interesting" scenarios