05-08-2011, 10:22 PM
(05-08-2011, 03:03 PM)Ian E G7OLT Wrote: If you're regularly working on mains-powered equipment, an RCD in the supply line. I'm old school and use a 500VA isolating transformer.
Just to pick up on that . . . .
I'm of that 'old school' too, but my approach combines the old school and the new . . .
I use a commercially-manufactured 1kVA - rated, self-contained, isolation transformer unit. It is fused, L and N, on its 230-v input. This input is fed, via a DPST switch, to the transformer primary. The incoming mains supply earth is connected to the metal of the case, etc. The secondary of the isol. transformer is centre-tapped, producing 120v - 0v - 120v. The two 120v lines are fed to a 30mA RCD that is mounted on the front panel of the unit's case. The centre tap of the sec'y. wndg. is not connected to the supply earth line, but it, and the two outgoing 120v lines (which are seperately fused) are fed to the sockets on the test bench, via a bench-mounted master switch (DPST). Thus my bench feed is 120v - 0v - 120v. And this bench feed is constantly monitored by a 300v FSD a.c. meter that is incorporated in the test bench display panel. All the sockets on the bench are conventional 3-pin 13 amp types, but are of the double-pole switching variety. This is obviously necessary, as opposed to SPST switching, since I am, in effect, providing a 120v bi-phase + 'earth' distribution system: I need to switch both phases.
In the event of a 120v - to - chassis fault on a piece of equipment on the bench, the RCD will obviously trip. But at the same time, the case / chassis of that item is not tied to the raw mains supply earth. Hence, I can connect the earth lead of one item of test kit - say a 'scope - to the chassis of an item under test quite safely.
Of course, I can't use this arrangement to provide a 120v / 0v supply, or as a power source for powering a.c. / d.c. equipment - each of which I rarely meet anyway. But even then, that is not a problem, since I then bring out the seperate 230v → 230v / 115v isolation transformer - and plug that in to a bench socket.
All sounds rather complicted - but, when presented as a cct. diag., it's really quite elementary & easy to follow. In essence, what I have produced is my own local miniature 120v bi-phase ring main that is electrically isolated from the raw mains supply and RCD protected.
For electrical safety testing - say post-repair - the test bench has a seperate single supply socket, complete with an RCD, that is fed direct from the raw a.c. mains.
And that arrangement, in concept, is relatively easy to duplicate anywhere else.
Al.
