110volt transformer question - UK Vintage Radio Repair and Restoration Discussion Forum

Bill · 21st Dec 2012, 4:52 pm

I have a 3kVA 240/110 isolating transformer. There is an earth screen between primary and secondary. There is no centre tap on the 110 side. Is it possible to create a centre tap (to earth) using resistors or something to make a 55-0-55 safety isolating transformer- would it comply with whatever standard is in place for this? It would be used for 110v power tools etc.
Thank you for your interest.
Bill

G8HQP Dave · 21st Dec 2012, 6:00 pm

I think the quick answer is no. It might be relatively safe to use, but if the standard calls for nothing greater than 55V then you need a 55-0-55 secondary.

Lucien Nunes · 21st Dec 2012, 6:03 pm

No, it needs a genuine centre-tap that can carry the full short-circuit current, so that no point in the circuit can rise to more than 55V WRT earth. If the transformer does not comply with BS3535, its insulation might also be inadequate.

Lucien

Herald1360 · 21st Dec 2012, 7:06 pm

If it's just to make 110V tools usable in a domestic setting, just ground one side of the secondary, that'll then just be the same as plugging the tool into the mains in the USA. If it's for use in a "building site" or other non domestic professional/commercial environment where regulations apply, then no, as above.

Bill · 21st Dec 2012, 11:40 pm

Thanks for your help, you have confirmed my own thoughts. Transformer now put back on a (heavy duty) shelf for some other future use.
Bill

G8UWM-MildMartin · 22nd Dec 2012, 3:04 am

Can you find out any more about the transformer, in particular what standards it was made to?
It may be suitable as an isolation transformer ("protection by electrical separation") for just one tool.
If not, I'd be happy using it as Chris (Herald 1360) mentioned, and even more so with an RCD (110V, obviously) on its output.

Radio Wrangler · 22nd Dec 2012, 9:57 am

********** CAUTION **********

RCDs and isolating transformers don't mix either way round.

If an RCD is put upstream of an isolating transformer, the transformer quite obviously breaks any possible loop involving ground. The RCD can never trigger and might just as well not be there. You still have the protection of the isolating transformer, but only the transformer

If the RCD is put downstream of an isolating transformer, something similar happens. To get an electric shock downstream of this RCD, you have to wind up bridging the two power conductors (words like live and neutral don't apply here). Perhaps your finger meets the two poles on a mains switch, or perhaps one power conductor has failed to chassis in the thing being repaired and you touch the other, or perhaps one power conductor goes to chassis and you touch the other. In all three cases the RCD sees the current on one power conductor coming back on the other. It passes what looks exactly like normal load current and does not trip. You still get the protection of the transformer, but the transformer alone. You have to touch two power conductors or else chassis and power conductor both at once to get a shock.

To make the RCD work downstream of an isolating transformer, you need an earth to power conductor connection UPSTREAM of it. This makes one power conductor neutral. If you personally conduct any current from the remaining power conductor, now definitely a 'live', to ground the RCD will trip properly. But with an earth connection on one side of the isolating transformer output, the isolating transformer is doing nothing except hum. You have the protection of the RCD, but of the RCD alone.

You always get the protection of one of the two items, but only one, not both.

The danger lies in thinking you're even safer with both connected, and therefore being prepared to take greater risks.

David

Refugee · 22nd Dec 2012, 10:19 am

My isolating transformer is on an RCD but only because a planned new socket outlet is not installed yet. It just feeds a Variac and one isolated socket direct.
It is as heavy as a bag of cement powder so it will go nowhere fast.

julie_m · 24th Dec 2012, 4:19 am

RCDs only work on circuits which are referenced to earth. Anything downstream of an isolating transformer will not be connected to earth (except parasitic coupling) and so a fault on the secondary side will not cause a current imbalance in the primary side. Of course, since the secondary is not earthed, touching only one wire will not give you a shock as there is no return path for the current; so an RCD is not necessary.

You could earth one end of the secondary, but then you would be losing the benefit of the isolation transformer!

21st Dec 2012, 4:52 pm	#1
Bill Pentode Join Date: Dec 2005 Location: Denton, Manchester, UK. Posts: 187	110volt transformer question I have a 3kVA 240/110 isolating transformer. There is an earth screen between primary and secondary. There is no centre tap on the 110 side. Is it possible to create a centre tap (to earth) using resistors or something to make a 55-0-55 safety isolating transformer- would it comply with whatever standard is in place for this? It would be used for 110v power tools etc. Thank you for your interest. Bill

21st Dec 2012, 6:00 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: 110volt transformer question I think the quick answer is no. It might be relatively safe to use, but if the standard calls for nothing greater than 55V then you need a 55-0-55 secondary.

21st Dec 2012, 6:03 pm	#3
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: 110volt transformer question No, it needs a genuine centre-tap that can carry the full short-circuit current, so that no point in the circuit can rise to more than 55V WRT earth. If the transformer does not comply with BS3535, its insulation might also be inadequate. Lucien

21st Dec 2012, 7:06 pm	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: 110volt transformer question If it's just to make 110V tools usable in a domestic setting, just ground one side of the secondary, that'll then just be the same as plugging the tool into the mains in the USA. If it's for use in a "building site" or other non domestic professional/commercial environment where regulations apply, then no, as above. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

21st Dec 2012, 11:40 pm	#5
Bill Pentode Join Date: Dec 2005 Location: Denton, Manchester, UK. Posts: 187	Re: 110volt transformer question Thanks for your help, you have confirmed my own thoughts. Transformer now put back on a (heavy duty) shelf for some other future use. Bill

22nd Dec 2012, 3:04 am	#6
G8UWM-MildMartin Heptode Join Date: Jan 2004 Location: Stockport, Greater Manchester, UK. Posts: 827	Re: 110volt transformer question Can you find out any more about the transformer, in particular what standards it was made to? It may be suitable as an isolation transformer ("protection by electrical separation") for just one tool. If not, I'd be happy using it as Chris (Herald 1360) mentioned, and even more so with an RCD (110V, obviously) on its output.

22nd Dec 2012, 9:57 am	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,871	Re: 110volt transformer question ******** CAUTION ******** RCDs and isolating transformers don't mix either way round. If an RCD is put upstream of an isolating transformer, the transformer quite obviously breaks any possible loop involving ground. The RCD can never trigger and might just as well not be there. You still have the protection of the isolating transformer, but only the transformer If the RCD is put downstream of an isolating transformer, something similar happens. To get an electric shock downstream of this RCD, you have to wind up bridging the two power conductors (words like live and neutral don't apply here). Perhaps your finger meets the two poles on a mains switch, or perhaps one power conductor has failed to chassis in the thing being repaired and you touch the other, or perhaps one power conductor goes to chassis and you touch the other. In all three cases the RCD sees the current on one power conductor coming back on the other. It passes what looks exactly like normal load current and does not trip. You still get the protection of the transformer, but the transformer alone. You have to touch two power conductors or else chassis and power conductor both at once to get a shock. To make the RCD work downstream of an isolating transformer, you need an earth to power conductor connection UPSTREAM of it. This makes one power conductor neutral. If you personally conduct any current from the remaining power conductor, now definitely a 'live', to ground the RCD will trip properly. But with an earth connection on one side of the isolating transformer output, the isolating transformer is doing nothing except hum. You have the protection of the RCD, but of the RCD alone. You always get the protection of one of the two items, but only one, not both. The danger lies in thinking you're even safer with both connected, and therefore being prepared to take greater risks. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done Last edited by Radio Wrangler; 22nd Dec 2012 at 10:01 am. Reason: phrasing of live chassis improved

22nd Dec 2012, 10:19 am	#8
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: 110volt transformer question My isolating transformer is on an RCD but only because a planned new socket outlet is not installed yet. It just feeds a Variac and one isolated socket direct. It is as heavy as a bag of cement powder so it will go nowhere fast.

24th Dec 2012, 4:19 am	#9
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: 110volt transformer question RCDs only work on circuits which are referenced to earth. Anything downstream of an isolating transformer will not be connected to earth (except parasitic coupling) and so a fault on the secondary side will not cause a current imbalance in the primary side. Of course, since the secondary is not earthed, touching only one wire will not give you a shock as there is no return path for the current; so an RCD is not necessary. You could earth one end of the secondary, but then you would be losing the benefit of the isolation transformer! __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.