Can someone please explain : centre tapped to earth on 110/240v portable isolation transformers. On the 110v side L&N are connected to earth, why dont the tranny trip on short cicuit? Why is it safer to use a tranny rather than 240v with a RCD?

Hi Uppeydog,

I assume from your question and terminology that you're located in British territory, or maybe Ireland.

The secondary of these transformers is 110V with the center tap earthed. That center-tap is the only point on the secondary which is earthed, so there can be no short-circuit. The two "outers" of the winding connect to the two current-carrying pins on the 110V sockets.

You thus have 110V for portable power tools, but each side of the circuit is only at 55V with respect to earth. This reduced voltage to earth lowers the shock risk.

The most sensitive RCD used here is generally a 30mA trip type. That's still enough current to be dangerous in some situations.

If American-style GFIs were used, which trip at around 6mA, then it would certainly be an improvement.

Thanks Paul, but this "centre Tap" is still bugging me. Sorry to be a nuisance, but could you do a wee sketch for me, of the secondary winding & this centre tap.
Cheers.

One very rough sketch:



(Dang! It's hard to draw transformer windings freehand with a mouse! )

Along with Paul's Schematic, maybe this item will be of help:



Other items directed towards 1Ø Transformers may be found at these following links:

1 Phase Trans. Schematics, Part 1

1 Phase Trans. Schematics, Part 2

See the Technical Reference section of ECN (This Site) for additional information and Schematics.

Scott 35

Aside from 120/240V, there is an onteresting North American variation similar to that used for British construction power. New in 1999, NEC Articles 640 and 530 ‘Technical Power’ for audio equipment uses an otherwise dedicated and isolated center-tap-grounded transformer secondary.

NEC ARTICLE 640 — Audio Signal Processing, Amplification, And Reproduction Equipment §640-7(b). “Technical Power” is a separately-derived system for reduction of 60Hz noise propagating through audio systems. [Center-tap grounded—termed “60/120V”. Center-tap conductor not permitted for serving loads—but locally, solidly-grounded only]

Referenced in 250-146(d) 640-22 640-7(c) 640-2 Technical Power System, and
530-73. Receptacles.
(2) All outlet strips, adapters, receptacle covers, and faceplates
shall be marked as follows:
     WARNING — TECHNICAL POWER
Do not connect to lighting equipment
For electronic equipment use only
60/120 volt 1ø ac
GFCI protected

Thanks for the better diagrams guys -- Threw mine together in about 5 minutes late last night!

Uppey,
Just to point out that the diagram in Scott's post of a 240V secondary with center-tap grounded is the standard arrangement for a domestic supply in North America.

With the U.K. 110V power-tool supplies, we only ever connect across the two outer "live" lines to get the full transformer secondary voltage.

In the American/Canadian domestic system, all three lines are brought into the house. Lights and outlets for most small appliances are then wired from one "hot" line to the neutral to run on 120V, while big appliances (range, dryer, etc.) are connected across the two hots to get 240V.

It's the same basic principle as the U.K. site supplies, though, with a center-tap ground on the transformer secondary.

Thanks Guy's,
I'm impressed with your replies,& with the explaination of the North American Domestic supply. I always wonderd what they meant, when "Hot wire" was mentioned!

Now one final Question....
UK..240v mains supply, with 30mA RCD: you get a 'Zap', the RCD trips, your ok.

110v secondary side of Tranny:
You hold one of the current carring pins, earth your other hand, you get a 55v 'Zap',
say you can't let go, RCD don't trip because it is on primary 240v supply.
Would the 55v break down your resistance & kill you! or would the current take the route to the centre tap to earth, because it has the least line of resistance?

If you held both current carrying pins, one in each hand, you get 110v 'zap', you can't let go, would this break down your resistance & kill you?

So what is the safest & why? ... a 240v with RCD, or 110v secondary supply? Please could you tell me the 'pros & cons'with each supply option.

Very much appreciated!

PS... Paul, have you ever tried errasing a sketch of windings with a rabbit!!

LOL!


It's used in the same sort of colloquial manner as Brits talk about the "Live" wire.

If you got yourself connected across one side of the supply and earth then, yes, you'd be hit with 55V, the current path being from one end of the transformer's secondary wiring through your body and earth back to the center-tap of the secondary winding.

55V could prove fatal under certain circumstances, but it's far less likely to kill than a 110V shock, everything else being equal.

The whole point of using the center-tapped arrangement is that by far the majority of electric shocks are sustained line-to-earth, i.e. somebody accidently touches an energized terminal while standing in a puddle, or leaning on earthed metalwork.

Shocks sustained line-to-line (or in a regular 240V UK system, line-to-neutral) are far less common.

If you used a simple 110V secondary winding with one end grounded, then a line-to-earth shock would be the full 110V. The center-tap system ensures that this most common method of getting a shock reduces that shock potential to 55V.

If you were "stuck" across 110V hand to hand for any appreciable time, then it could very well prove fatal. A hand-to-hand shock is generally one of the most serious ways to be shocked, as the current passes directly through the chest where it can affect the heart and respiratory system.

I think this could be very much open to argument. I always like to point out that a 30mA RCD is not an absolute guarantee against electrocution, despite the fact that many people are left with that impression from the way they are marketed these days. A sustained shock at just below 30mA could certainly be fatal to some people in certain circumstances.

One other point to keep in mind when looking at one system versus the other is that the 110V CTE arrangement for power tools was introduced long before 30mA or even 100mA RCD protection was commonplace.

30-odd years ago, the earth leakage protection, where provided at all, would be more likely to be in the order of 500mA. A 240V shock which results in a current of that magnitude through the chest for anything but the briefest of periods is almost certain to result in ventricular fibrillation.


[This message has been edited by pauluk (edited 07-12-2004).]

Thanks Paul,
Very good reply, well explianed.
Very true about the RCD. Some people assume
because they are using an RCD they can do all sorts of strange things & if anything goes wrong the RCD will trip. I think it should be made clearer that the device will only trip with a live to earth fault, & will not trip with a live to neutral fault.
I NOW fully understand the CTE principle
so its obvious you could not have a similar device on the secondary out-put side due to the CTE. Is there a safty device you can use on this type of circuit?

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.