"Sammo" wrote | However, if the cable has resistence then presumably current capacity | (ignoring voltage) gets reduced. I am thinking of W = I^2 * R.
No, because that resistance - and hence the heating effect - is spread along the length of the cable. If 1m of cable is rated for 10A then 100m of cable will also be rated for 10A. You will probably need a thicker cable to reduce the voltage drop, but that is not the same as the current-carrying capacity of the cable.
| I have got a plug-in RCD device but I know nothing about earth fault | loops. It is similar to the one in this illustration. |
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| Where should my RCD device be plugged in to best avoid the problem of | earth fault loops: | (1) At the mains supply end where the first extension goes into the | mains socket.
Here, because the RCD protection must be applied to the whole extension flex, because it is in effect a portable appliance.
| (2) In the middle of the two 30m extensions.
No, because that would leave the first flex (or more specifically - someone handling it) unprotected.
| (3) At the far end of the two extensions where the appliciance I am | using is plugged in?
No, same reason.
If you were wiring a *permanent circuit* then RCD protection could usually be applied at the load end, because fixed wiring does not need such a high standard of protection.
Note, however, that using an RCD plug on only one extension lead raises the possibility that someone might plug the extension leads togther the wrong way round, with the result that the first flex would be unprotected. The way round this is to have an RCD plug on both leads -- this will do no harm, but either or both may trip in the event of a fault -- or use an RCD socket.
The point remains -- why are you contemplating using 60m extension cables?
Owain