ECN Forum Forums Technical Discussion Electrical Theory and Applications CTE in 240/ 110v Transformers

Actually there's no reason why earth-leakage protection could not be put on the secondary side of these tool transformers. The center-tap earth arrangement doesn't in any way preclude RCD use (the RCD would need to be one designed for 110V of course).

The RCD works on the principle of comparing the current in two conductors and tripping if they differ by more than a set amount. There's no need for one of those conductors to be earthed (a neutral).

If you wired something like an American GFI (Ground Fault Interrupter) on the secondary side across the two hot/live lines you'd then have a 110V supply, with no more than 55V to earth, and with earth-leakage protection that would trip on about 6mA.

Thanks Paul, I'll look into this Ground Fault Interupter to suit a 110v supply.

hello uppey
we hafe a GFCI for 110 he rin the US. it is a code required item in all of the bathrooms and for a minamum of 2 circuits in the kitchens, we use a receptacle and protect other outlets down line from it or a breaker in the panel.

Howdy Sparkystudent,
Sounds like a good system. I'm going to have to fly over to U.S.A. for a wet shave ....just to test the system.. of course!!
I wonder why the U.K. system wasn't set up the same way?

I think it's really down to the slightly different way that our systems developed.

In the U.S.A., all supplies have the neutral grounded/earthed at the service entrance, so historically ground-fault protection was not needed to clear a fault such as line-to-earth short, as the fuse or circuit breaker could be relied upon. In British terms, the U.S. arrangament is equivalent to PME.

The addition of sensitive ground-fault protection has come about gradually over the last 30-odd years mainly as a shock-prevention measure, by adding 6mA GFIs to those individual circuits deemed to pose the greatest risk (i.e. first bathrooms, then kitchens, and so on).

In the U.K., however, the principles of ground-fault protection evolved along slightly different lines, as we have many rural systems running under the TT earthing arrangement, where there is no neutral-earth bond at the premises and the earth itself forms part of the fault-current path.

Earth-leakage circuit breakers were therefore required simply to clear a line-to-earth fault, as the loop impedance would not be low enough for a normal fuse or circuit breaker to trip under these conditions.

Thus we had (and still have) many houses where there is a single ELCB (or its modern RCD equivalent) providing such protection for the whole house.

As closer-protection has become more sought-after in recent years, the natural tendency has been to just make that main RCD more sensitive, first down to 100mA and now to the common 30mA level.

Individual branch-circuit protection could be provided here, American-style, but the lack of demand for such breakers makes such a panel pretty expensive by comparison.

The compromise situation of split-bus panels and only some circuits run under a "sub-main" RCD is one result of that. Of course, in areas with TT earthing, the only way to split to the bus is to then use two or more main RCDs.

Trying to put RCD protection for multiple circuits down to much below 30mA poses considerable problems, and even 30mA is awkward, yet that level of current is still quite high enough to prove dangerous in some shock conditions.

Personally, I think the American approach of 6mA GFIs on individual circuits is a far better method.

[This message has been edited by pauluk (edited 07-17-2004).]

Thanks guys....really impressed with your answers.....Cheers!!
For being so smart here is an extra RCD Problemo...
A friend of mine is opening a small commercial business. He asked me for a 5Mtr. extension with a 16Amp (Blue) Socket on one end & a standard 13Amp 3 Pin Plug on the other end. I told him it was illeagal & dangerous.
He has an Installation electrician working for him, & he wants him to install a scond-hand commercial freezer which has a 16Amp (Blue)Plug Attatched. He wants to connect it to a 13Amp wall RCD for safety!!! which is 8 Mtrs. away.
I've told him if his freezer draws over 11 Amps he will keep blowing fuses, melt the RCD unit, loose all his freezer stock & possibly burn the place down!!!
I've told him to tell his Sparky, to run a seperate supply from a 20Amp MCB to a 16Amp(Blue) wall socket with an Integral RCD Also to make sure the freezer cable is no more than 3 Metres long.
Would this be the way to go within the Commercial Electrical Regulations? Also whats the limit on Appliance Cable length
within the regulations? Better he knows now before the place gets it's inspection before opening!!!

hello im trying to get this too, can anyone tell me if you protected your armoured cable with an rcd that is feeding your tx and there was a fault on the 110 yellow cable of the tx, would the rcd operate at source????, i know l and n are seperated so i dont think it would......also if you had an rcd at the primary side of the tx i.e you terminated your aroured into an rcd would this operate as this is also getting me...thanks

Good question!
The puzzler in this case is the fact that the secondary is actually earthed. If it weren't, a secondary earth fault would simply cause one of the secondary conductors to assume 0 V (earth) potential and the other would shift to the transformer's regular output voltage. However with an earthed secondary I think a primary RCD COULD trip, but I'm not sure. Electrical theory isn't exactly my main field these days and centre-tapped transformers are quite special beasts anyway.