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#129075 - 07/10/04 08:39 AM CTE in 240/ 110v Transformers
Uppeydog Offline
Member

Registered: 07/09/04
Posts: 64
Loc: Loch Lomond
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?
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#129076 - 07/10/04 09:28 AM Re: CTE in 240/ 110v Transformers
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Hi Uppeydog,

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

 Quote:
On the 110v side L&N are connected to earth, why dont the tranny trip on short cicuit?

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.

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#129077 - 07/10/04 11:01 AM Re: CTE in 240/ 110v Transformers
Uppeydog Offline
Member

Registered: 07/09/04
Posts: 64
Loc: Loch Lomond
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.
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#129078 - 07/10/04 05:11 PM Re: CTE in 240/ 110v Transformers
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
One very rough sketch:



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

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#129079 - 07/10/04 06:49 PM Re: CTE in 240/ 110v Transformers
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
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
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#129080 - 07/10/04 08:08 PM Re: CTE in 240/ 110v Transformers
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
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

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#129081 - 07/11/04 03:10 AM Re: CTE in 240/ 110v Transformers
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
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.

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#129082 - 07/11/04 06:12 AM Re: CTE in 240/ 110v Transformers
Uppeydog Offline
Member

Registered: 07/09/04
Posts: 64
Loc: Loch Lomond
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!!
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#129083 - 07/12/04 03:02 PM Re: CTE in 240/ 110v Transformers
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
LOL!

 Quote:
I always wonderd what they meant, when "Hot wire" was mentioned!


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

 Quote:
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 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.

 Quote:
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?

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.

 Quote:
So what is the safest & why? ... a 240v with RCD, or 110v secondary supply?

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).]

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#129084 - 07/13/04 02:09 PM Re: CTE in 240/ 110v Transformers
Uppeydog Offline
Member

Registered: 07/09/04
Posts: 64
Loc: Loch Lomond
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?
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