RCD testing using an isolation transformer. - UK Vintage Radio Repair and Restoration Discussion Forum

Alistair D · 8th Apr 2014, 11:01 pm

http://www.tortech.com.au/isolation-...or-rcd-testing

Can anyone shed some light on how these transformers are wired differently to a normal isolation transformer?

Al

Radio Wrangler · 8th Apr 2014, 11:07 pm

That's a secret!

They carefully give you a schematic of what their transformer ISN'T, and give no info on what theirs IS. But they do say that it's very good.

I'd want to know a lot more than nothing before I'd trust my safety to it.

Bet all the snakes in their vicinity don't squeak.

David

Alistair D · 8th Apr 2014, 11:17 pm

Indeed David, I have spent a few hours searching the net for an answer with no luck. The various references to to this configuration all end in dead ends. It cannot be that difficult to work it out yet it beats me.

Al

Dekatron · 9th Apr 2014, 12:06 am

Isn't it just a usual "Balanced AC power" design like the one in this thread: http://www.hometheatershack.com/foru...c-power-2.html where the center tap of the secondary winding is connected to common ground/earth?

GMB · 9th Apr 2014, 9:02 am

Surely it just means that they have joined the N and E together on the outlet socket instead of either leaving E floating or joining it to the input earth.

I think the confusion here is that when they talk about "isolation transformer" they don't mean the thing with laminations and windings, they mean the whole unit in a box with flex and an outlet socket.

Radio Wrangler · 9th Apr 2014, 9:18 am

It probably is, but they're not telling people what it is, and then expect those people to trust themselves to it.

I've got a few 'tool transformers' and their 55-0-55 output is not a secret.

David

RogerWalker · 9th Apr 2014, 11:55 am

Their circuit diagram [of what it isn't] looks decidedly dodgy and with the neutral earthed, would trip the RCD in my house for sure. None of it makes much sense to me - mirrors and quite a lot of smoke!

Roger Walker

Alistair D · 9th Apr 2014, 3:34 pm

A bit of background.

After a bit of discussion with a friend about the shock reduction risk of using an isolation when repairing AC/DC sets we concluded that the risk is back the non isolated level as soon as an earthed instrument is connected to the chassis. That led me think about adding a plug top RCD to the transformer output.

I tried connecting the N and E together on my MEGGER CBT3 and plugged it into the socket attached to the RCD. It would not trip at any current. I even tried a 60W bulb L to E(N to E still joined) The bulb glowed at full strength and no trip.
I did find this thread which agrees with my results.

http://www.theiet.org/forums/forum/m...threadid=31769

The problem, as I see it, is that with N and E joined is that the current passed to Earth by the tester goes back the Neutral and thus no imbalance is created.

Dekatron; It would seem that that may be the answer although absolutely no use for a bench isolation transformer. I fact I would not call that an isolation transformer since both the L and N terminals have a potential of 110V above earth and I certainly would want to know about that.

Roger; Agreed. My main RCD tripped out when I plugged the modified RCD tester lead into a non isolated socket.

I still have a few ideas to play with and will report back later.

Two further points. The transformer's output earth is connected to the input earth.
The test button, as expected, works fine when the RCD is plugged into the transformer.

Al

GMB · 9th Apr 2014, 4:38 pm

When running on isolated supply, the earthed test equipment does indeed create a shock risk when connected, and as you found an RCD doesn't help. This is a hazard of isolation and a reason to be very careful.

The explanation is clear: if you touch the isolated live in that situation the current flows through your body to physical earth, then up wherever the real mains earth is physically earthed, then back along the mains earth wire to your equipment, then into the chassis where that is connected and so completing the "correct" path and thus the isolated RCD is not interested to save you.

Specifically for testing RCDs, if the isolation transformer output joins N to E and the RCD is plugged into that then current that shorts inside the equipment between live and earth will trip the RCD.

The RCD test button always works because in order not to trip a cascade of RCDs it must in fact leak current from the live output to the neutral input and not involve the earth connections at all.

dave cox · 9th Apr 2014, 5:34 pm

If you were using a CT isolation transformer (120-0-120) to test an RCD you do not actually need to connect the CT to physical earth as you just need to bleed current from the RCD output to the CT and the device should trip as there is now a current imbalance. Other RCD's on the primary side see no imbalance so don't trip.

Nailing the CT to earth gives you 2 chances out of 2 of giving yourself a 120V shocks as apposed to 1 chance out of 2 of getting 240V. With no CT earth you SHOULD just pull one end down to ground with a tingle

dc

woodchips · 9th Apr 2014, 8:00 pm

Try wiring the isolating transformer such that the live out goes to the RCD live in and the transformer neutral goes to the RCD neutral in. The sneaky bit is to then wire the earth from the test bench etc to the connection between the transformer neutral and the RCD neutral in. Now if a 30mA leakage exists from the live to earth then it goes directly to the transformer neutral, creating the 30mA imbalance in the RCD to trip it.

As GMB says, the test button doesn't trip all the RCDs in the series circuit. There must be a third winding around one leg in the RCD such that pressing the button simply puts a 30mA current through this winding and hence trips the RCD. Obviously it doesn't have to be 30mA, just has the same effect as 30mA. So, if you measure the resistance between the incoming live and neutral pins it will be infinity, pressing the test button will produce a low DC resistance. Could be just a few ohms, the extra winding will have impedance to AC, not to DC. Can't find an RCD to test, just theory at the moment!

Alistair D · 9th Apr 2014, 9:37 pm

Woodchips, the way a test button on an RCD plug works is by connecting a resistor between the input side of the neutral current measurement winding and the output side of the live current measurement winding. That way it will still work if there is an earth fault on the socket.

I will draw the circuit out tomorrow and try it out.

Al

glowinganode · 9th Apr 2014, 10:02 pm

If I may, I'd like to offer this theory.
Imagine you are a bench electrical test engineer. Your bench is fed from a 30 mA RCD protected supply "for safety".
Part of your job requires you to test RCD's on portable appliances.
However you find your RCD tester occasionally trips your bench RCD, especially when doing an 5I test.
This is because the differential fault current flows through both the RCD under test, and the bench supply RCD.
I could not download the circuit, but I understand the primary is floating, and the secondary bonded at one end to earth.
By using this transformer, the differential fault current would only flow through the RCD under test.
The primary current is balanced and therefore does not now trip the bench supply RCD.

Rob.

8th Apr 2014, 11:01 pm	#1
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	RCD testing using an isolation transformer. http://www.tortech.com.au/isolation-...or-rcd-testing Can anyone shed some light on how these transformers are wired differently to a normal isolation transformer? Al __________________ I won't tell you how I discovered that.

8th Apr 2014, 11:07 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,871	Re: RCD testing using an isolation transformer. That's a secret! They carefully give you a schematic of what their transformer ISN'T, and give no info on what theirs IS. But they do say that it's very good. I'd want to know a lot more than nothing before I'd trust my safety to it. Bet all the snakes in their vicinity don't squeak. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

8th Apr 2014, 11:17 pm	#3
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: RCD testing using an isolation transformer. Indeed David, I have spent a few hours searching the net for an answer with no luck. The various references to to this configuration all end in dead ends. It cannot be that difficult to work it out yet it beats me. Al __________________ I won't tell you how I discovered that.

9th Apr 2014, 12:06 am	#4
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: RCD testing using an isolation transformer. Isn't it just a usual "Balanced AC power" design like the one in this thread: http://www.hometheatershack.com/foru...c-power-2.html where the center tap of the secondary winding is connected to common ground/earth? __________________ Martin, Sweden

9th Apr 2014, 9:02 am	#5
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,094	Re: RCD testing using an isolation transformer. Surely it just means that they have joined the N and E together on the outlet socket instead of either leaving E floating or joining it to the input earth. I think the confusion here is that when they talk about "isolation transformer" they don't mean the thing with laminations and windings, they mean the whole unit in a box with flex and an outlet socket.

9th Apr 2014, 9:18 am	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,871	Re: RCD testing using an isolation transformer. It probably is, but they're not telling people what it is, and then expect those people to trust themselves to it. I've got a few 'tool transformers' and their 55-0-55 output is not a secret. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

9th Apr 2014, 11:55 am	#7
RogerWalker Hexode Join Date: Sep 2005 Location: Powys, Mid-Wales, UK. Posts: 289	Re: RCD testing using an isolation transformer. Their circuit diagram [of what it isn't] looks decidedly dodgy and with the neutral earthed, would trip the RCD in my house for sure. None of it makes much sense to me - mirrors and quite a lot of smoke! Roger Walker

9th Apr 2014, 3:34 pm	#8
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: RCD testing using an isolation transformer. A bit of background. After a bit of discussion with a friend about the shock reduction risk of using an isolation when repairing AC/DC sets we concluded that the risk is back the non isolated level as soon as an earthed instrument is connected to the chassis. That led me think about adding a plug top RCD to the transformer output. I tried connecting the N and E together on my MEGGER CBT3 and plugged it into the socket attached to the RCD. It would not trip at any current. I even tried a 60W bulb L to E(N to E still joined) The bulb glowed at full strength and no trip. I did find this thread which agrees with my results. http://www.theiet.org/forums/forum/m...threadid=31769 The problem, as I see it, is that with N and E joined is that the current passed to Earth by the tester goes back the Neutral and thus no imbalance is created. Dekatron; It would seem that that may be the answer although absolutely no use for a bench isolation transformer. I fact I would not call that an isolation transformer since both the L and N terminals have a potential of 110V above earth and I certainly would want to know about that. Roger; Agreed. My main RCD tripped out when I plugged the modified RCD tester lead into a non isolated socket. I still have a few ideas to play with and will report back later. Two further points. The transformer's output earth is connected to the input earth. The test button, as expected, works fine when the RCD is plugged into the transformer. Al __________________ I won't tell you how I discovered that. Last edited by Alistair D; 9th Apr 2014 at 3:43 pm. Reason: Added further text.

9th Apr 2014, 4:38 pm	#9
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,094	Re: RCD testing using an isolation transformer. When running on isolated supply, the earthed test equipment does indeed create a shock risk when connected, and as you found an RCD doesn't help. This is a hazard of isolation and a reason to be very careful. The explanation is clear: if you touch the isolated live in that situation the current flows through your body to physical earth, then up wherever the real mains earth is physically earthed, then back along the mains earth wire to your equipment, then into the chassis where that is connected and so completing the "correct" path and thus the isolated RCD is not interested to save you. Specifically for testing RCDs, if the isolation transformer output joins N to E and the RCD is plugged into that then current that shorts inside the equipment between live and earth will trip the RCD. The RCD test button always works because in order not to trip a cascade of RCDs it must in fact leak current from the live output to the neutral input and not involve the earth connections at all.

9th Apr 2014, 5:34 pm	#10
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: RCD testing using an isolation transformer. If you were using a CT isolation transformer (120-0-120) to test an RCD you do not actually need to connect the CT to physical earth as you just need to bleed current from the RCD output to the CT and the device should trip as there is now a current imbalance. Other RCD's on the primary side see no imbalance so don't trip. Nailing the CT to earth gives you 2 chances out of 2 of giving yourself a 120V shocks as apposed to 1 chance out of 2 of getting 240V. With no CT earth you SHOULD just pull one end down to ground with a tingle dc

9th Apr 2014, 8:00 pm	#11
woodchips Octode Join Date: May 2010 Location: Grantham, Lincolnshire, UK. Posts: 1,174	Re: RCD testing using an isolation transformer. Try wiring the isolating transformer such that the live out goes to the RCD live in and the transformer neutral goes to the RCD neutral in. The sneaky bit is to then wire the earth from the test bench etc to the connection between the transformer neutral and the RCD neutral in. Now if a 30mA leakage exists from the live to earth then it goes directly to the transformer neutral, creating the 30mA imbalance in the RCD to trip it. As GMB says, the test button doesn't trip all the RCDs in the series circuit. There must be a third winding around one leg in the RCD such that pressing the button simply puts a 30mA current through this winding and hence trips the RCD. Obviously it doesn't have to be 30mA, just has the same effect as 30mA. So, if you measure the resistance between the incoming live and neutral pins it will be infinity, pressing the test button will produce a low DC resistance. Could be just a few ohms, the extra winding will have impedance to AC, not to DC. Can't find an RCD to test, just theory at the moment!

9th Apr 2014, 9:37 pm	#12
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: RCD testing using an isolation transformer. Woodchips, the way a test button on an RCD plug works is by connecting a resistor between the input side of the neutral current measurement winding and the output side of the live current measurement winding. That way it will still work if there is an earth fault on the socket. I will draw the circuit out tomorrow and try it out. Al __________________ I won't tell you how I discovered that.

9th Apr 2014, 10:02 pm	#13
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: RCD testing using an isolation transformer. If I may, I'd like to offer this theory. Imagine you are a bench electrical test engineer. Your bench is fed from a 30 mA RCD protected supply "for safety". Part of your job requires you to test RCD's on portable appliances. However you find your RCD tester occasionally trips your bench RCD, especially when doing an 5I test. This is because the differential fault current flows through both the RCD under test, and the bench supply RCD. I could not download the circuit, but I understand the primary is floating, and the secondary bonded at one end to earth. By using this transformer, the differential fault current would only flow through the RCD under test. The primary current is balanced and therefore does not now trip the bench supply RCD. Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.