Bench Isolation Transformer

[tom66]

I have an isolated transformer with the earth mains connected to the output socket. Only live and neutral are isolated. This makes perfect sense to me for power supplies as in the majority of cases, the earth is bounded to the common (return/ground) and you can safely probe across mains live and neutral (with a scope probe, for example, one end earthed) -- you just end up connecting the isolated neutral to earth -- which should be safe as it's isolated.

I haven't done this though, I'm too much of a coward. Stupid idea? Am I still protected by RCD?

[nctnico]

What if I connect a GFCI on the isolated side of my transformer? If I am grounding the DUT for measurement I still have some protection with a GFCI.

No, you won't. Just forget about the isolation transformer. Using it on a test bench is prone to errors and gives a false sense of safety. The posting above just proved my point... Working with high voltages must be avoided as much as possible. If it is not possible you should think three times about what you are doing and assume you can't touch anything. Darwin awards can be awarded post-mortem as well.

[ivan747]

If both the live and neutral coming out of the transformer are floating relative to ground, what alternative path could the current take that would trip the GFCI?

Let's say I want to connect probe's ground clip to the primary side, that's grounding one of those lines. Another possibility would be connecting a USB programmer to the circuit.

[Monkeh]

If both the live and neutral coming out of the transformer are floating relative to ground, what alternative path could the current take that would trip the GFCI?

Let's say I want to connect probe's ground clip to the primary side, that's grounding one of those lines. Another possibility would be connecting a USB programmer to the circuit.

But there's no path for the current except through the RCDs coil. So it won't trip.

No, you won't. Just forget about the isolation transformer. Using it on a test bench is prone to errors and gives a false sense of safety.

And RCDs give a false sense of safety, too.

[David_AVD]

I thought the work site transformers in the US used an isolated split secondary with the centre tap grounded? (55V AC either side of ground)

I thought I had started having a good understanding of these things now you say the secondary has a grounded central tap ... Is the transformer still isolated in this case? Because its secondary is not galvanically isolated from the earth.

My recollection of this is a bit rusty, but I think the idea (on work sites) was to make it so the worst case electrical hazard was 55V with respect to mains earth.  I guess it's not an isolated secondary as such - more so that the Active and Neutral have been shifted to swing around earth potential, not on top of it.

[vk6zgo]

What if I connect a GFCI on the isolated side of my transformer? If I am grounding the DUT for measurement I still have some protection with a GFCI.

No, you won't. Just forget about the isolation transformer. Using it on a test bench is prone to errors and gives a false sense of safety. The posting above just proved my point... Working with high voltages must be avoided as much as possible. If it is not possible you should think three times about what you are doing and assume you can't touch anything. Darwin awards can be awarded post-mortem as well.

Hundreds of thousands of Techs work with Mains level voltages every day,& get around any danger by using commonsense safety rules similar to these:-

(1)
 If you are working on the non-isolated side  of a switch mode power supply,& you need to look critically at the waveforms involved using an Oscilloscope,you should use an isolation transformer so that you remove the risk of accidentally blowing up your 'scopes internal earth connections.

There is a"but" though,and a big one!

If you connect the earth clip of your 'scope probe to one side of the isolation transformer secondary,that side becomes connected to earth,so you need to regard anything connected to the other side as "hot" w.r.t.earth,& capable of giving you a serious shock.---As this new impromptu circuit is still isolated from the house wiring,your RCD is not in circuit,& won't work.

DMM's are "floating" w.r.t. earth,& the isolation transformer offers good protection for readings using these devices.

(2)
If you can,set up any probe connections before you turn the equipment on,then you are not in direct contact if you stuff anything up.

(3)
If you are working on equipment with a simple transformer power supply,you will normally only have to check for the presence of  Mains power at the primary.
You can use your Oscilloscope.Don't connect your probe earth to anything,clip it back out of the way,or better detach it.
Your earth return is now via the 'scope's  Mains earth,which will allow an accurate enough display for this purpose.

In any case,you can just hang your DMM (Switched to AC & with the leads in the correct sockets),across the primary,to see if you have Mains available.

This trick will also give you a fair idea of what is happening on the non isolated side of a switchmode supply,but obviously,high frequency components of the switching waveforms will not be reproduced well,due to the long earth return circuit.

Mains,& other voltages of similar levels can be hazardous,but you can minimise any danger by being careful,just as,(I hope),you do when in the much more dangerous activity of driving.

[Zero999]

Safety aside, you may want to consider one with two 120V secondary windings so you can use it to test European equipment.

[nctnico]

No, you won't. Just forget about the isolation transformer. Using it on a test bench is prone to errors and gives a false sense of safety. The posting above just proved my point... Working with high voltages must be avoided as much as possible. If it is not possible you should think three times about what you are doing and assume you can't touch anything. Darwin awards can be awarded post-mortem as well.

So what you're saying is that we should all stick to battery operated circuits?

Look its good to have respect for something that can bite you but…you are getting a little paranoid. As a technician, technologist or engineer you should be trained to work safely with potentially hazardous voltages. The same applies for people working on assembly lines that have the potential to maim or kill and anyone of a thousand different jobs that are potentially hazardous such as an electrician. Would you suggest they quit working because they may get hurt?

You are twisting my words. What I'm saying is that it is best to avoid having to measure in mains connected circuitry. When I was an intern at a mainframe repair department and was working on big 5V >300A power supplies they showed me some neat tricks to avoid having to work on a PSU with lethal voltages. One of them was to supply the primary control circuit from a bench supply. Then you can measure everything you want in a very safe way.

[Rick]

If you connect the earth clip of your 'scope probe to one side of the isolation transformer secondary,that side becomes connected to earth,so you need to regard anything connected to the other side as "hot" w.r.t.earth,& capable of giving you a serious shock.---

So according to what you say, similarly the isolation transformer with its secondary central tap connected to mains earth (we were talking about that config earlier in this thread) has its two "floating" outputs live... as the central tap is connected to the two outputs through the secondary windings. That is you lose the benefit of using an isolation transformer (having the outputs floating with regard to earth). This is what I was trying to understand in my earlier posts.

[Rick]

I thought the work site transformers in the US used an isolated split secondary with the centre tap grounded? (55V AC either side of ground)

I thought I had started having a good understanding of these things now you say the secondary has a grounded central tap ... Is the transformer still isolated in this case? Because its secondary is not galvanically isolated from the earth.

My recollection of this is a bit rusty, but I think the idea (on work sites) was to make it so the worst case electrical hazard was 55V with respect to mains earth.  I guess it's not an isolated secondary as such - more so that the Active and Neutral have been shifted to swing around earth potential, not on top of it.

But then if you touch now the two outputs around the central tap with a screwdriver, the neon will be on, right? Other than reducing the voltage in case of an electric shock it offers no protection when you touch the floating terminals separately as in the case of an isolated transformer. I hope I finaly got  it

[Rick]

One of them was to supply the primary control circuit from a bench supply. Then you can measure everything you want in a very safe way.

You mean you were using a 220V source (something like a power inverter?) to feed the primary circuit? You need in this case an extra equipment too and it will have its own isolation inside I guess.

[vk6zgo]

I thought the work site transformers in the US used an isolated split secondary with the centre tap grounded? (55V AC either side of ground)

I thought I had started having a good understanding of these things now you say the secondary has a grounded central tap ... Is the transformer still isolated in this case? Because its secondary is not galvanically isolated from the earth.

My recollection of this is a bit rusty, but I think the idea (on work sites) was to make it so the worst case electrical hazard was 55V with respect to mains earth.  I guess it's not an isolated secondary as such - more so that the Active and Neutral have been shifted to swing around earth potential, not on top of it.

But then if you touch now the two outputs around the central tap with a screwdriver, the neon will be on, right? Other than reducing the voltage in case of an electrical shock it offers no protection when you touch the floating terminals separately as in the case of an isolated transformer. I hope I finally got  it

I suppose 55V is not as nasty as 110V,but you have just doubled the chance of a shock,compared to a straight mains feed on a 110V system.
You also have the same chance,however small,of getting across both sides of the secondary as you have with an untapped transformer.

As far as I know,Australian work sites all use straight 240V double insulated equipment,operated from power boxes with
RCDs,-----or,in some instances use pneumatic tools instead.

Remember,to Tradespeople on work sites,the Electrical equipment is only one possibility for injury,& a pretty small one at that.
They are also concentrating on their real job,& expect their tools to be safe.
EEs & Electrical & Electronics Tradespeople, even though they are in the proximity of bare live conductors,are concentrating on the Electrical equipment,so are far less likely to be distracted,& get zapped,

I worked in Broadcasting & other high power RF jobs for around 40 years.
On big Transmitters,the HT supply can be around 10kV at several Amps,& the power supplies are 3 phase 415V.

In that time,I knew,& worked with,five people who were accidentally killed.

Of them,two died in falls,two in air crashes, ( one in a Pitt's Special,& the other,in an Airliner),& one was washed off a ledge while fishing,but not one died from Electrocution!

I think nctnicowas making the point that a lot of the control circuitry in switchmode supplies,even though they may be elevated to 110V or 240V in normal use,actually operate from quite low voltages,which can be externally supplied to test their operation.
This is a good idea,& is quite widely used,but ultimately the Supply will have to be tested in its normal operating condition.

[Monkeh]

I suppose 55V is not as nasty as 110V,but you have just doubled the chance of a shock,compared to a straight mains feed on a 110V system.
You also have the same chance,however small,of getting across both sides of the secondary as you have with an untapped transformer.

Not only does it reduce the chance of a shock (55V is rather less likely to get through you than 110V or 240V), but it also limits the PFC.

[vk6zgo]

I suppose 55V is not as nasty as 110V,but you have just doubled the chance of a shock,compared to a straight mains feed on a 110V system.
You also have the same chance,however small,of getting across both sides of the secondary as you have with an untapped transformer.

Not only does it reduce the chance of a shock (55V is rather less likely to get through you than 110V or 240V), but it also limits the PFC.

How does it reduce the chance of a shock?
Perhaps the severity,but you now have two active conductors w.r.t earth,which sounds like twice the likelihood to me!

OK,at 55V,it probably won't kill you,but is still going to be a shock,which implies secondary dangers if you are using a large industrial type tool,as any involuntary movement may injure you or someone else.

The non tapped secondary,combined with double insulation in the tools should minimise any possibility of shock,as indeed the latter should,in any case.

How does it help the PF?
Do they use 55V tools on either side of the tap,or 110V ones across the whole secondary ?

In any case,an earthed centre tap on a bench isolation transformer is unnecessary,& as far as I know,not used.

[T4P]

Yep, inverters. I've seen the 1kW ones ... 20+ kgs for a more efficient one
A thing to note : They are real man's inverters the ones you can change frequency and even change the waveform(i think)!
Variable voltage on top of that ...

[Monkeh]

How does it reduce the chance of a shock?
Perhaps the severity,but you now have two active conductors w.r.t earth,which sounds like twice the likelihood to me!

55V is by no means sure to pass through a man wearing boots.

How does it help the PF?

PFC as in Prospective Fault Current, not Power Factor Correction.

[funkimunky]

do you get 240v isolated power transformers for using in the uk to isolate mains powered equipment?

[Monkeh]

do you get 240v isolated power transformers for using in the uk to isolate mains powered equipment?

Well, yes.

[Rick]

So when we say the outputs of an isolation transformer are floating above earth we are basically saying that, separately taken, they present no potential... is that right? They are at the same potential as the earth is, normally zero volt potential. Otherwise current would flow through the body of the person touching one terminal of a real isolated transformer (with no central tap). But this is not the case.

[lewis]

There are isolation transformers and there are isolation transformers. Don't get confused between the two!



This isolation transformer is designed with the OP's application in mind (http://uk.rs-online.com/web/p/safety-site-transformers/4368805/). It does NOT have a centre-tapped earth. I've just opened mine up to check. The output winding is completely floating, you can short the output live to earth, or the output neutral to earth with no problems, and you can even connect the secondary in series with the primary to get 480V if you really want. An RCD (or GFI if you prefer) on either end of the transformer will do nothing. This is good in some respects because if you touch the output live, you don't get a belt (in theory - I'll get to that in a moment). But if you're testing something that has an earth leakage fault, maybe an SMPS with a leaky cap in the EMI filter, you won't notice.

However, caveat emptor, you can still get a small shock if you touch the transformer secondary (live or neutral) and earth. Why? Interwinding capacitance (IWC). I measured 76V between output neutral and earth, and 38V between output live and earth with a fluke 87-V (10M input impedance?), and shorting to earth with the uA AC range I measured 36uA and 4uA respectively. Not a huge amount, but enough to feel it if you have wet hands. (Yes, I tried it). The IWC will vary from unit to unit, probably has something to do with temperature and will cause a higher leakage if you have harmonics on the mains. If you're trying to make sensitive measurements on an SMPS this IWC can give erroneous results. But it's much better than having the full mains across you. If you can find a transformer with an earthed electrostatic screen between primary and secondary, IWC is not an issue, but you still have leakage capacitance between secondary and earth.




This isolation transformer is different (http://uk.rs-online.com/web/p/safety-site-transformers/2604272/). It takes 240V down to 110V for the 110V tools we have to use in the UK on building sites. It DOES have a centre tapped earth - the transformer secondary is wired as 55-0-55, with the 0 connected to mains earth. So between output 'live' and earth there's 55V and between output 'neutral' and earth there's another 55V.

Someone earlier mentioned that because there are two conductors carrying 55V you're twice as likely to get a shock. Maybe so, but it is FAR more likely that the shock will occur between one of the conductors and earth than between live and neutral. Perhaps you're standing in a muddy puddle and you cut through or nick the cable. Or maybe you're holding a Hilti drill (earthed) and touch a dodgy connection with the other hand. Remember these are for building sites. The shock is therefore limited to 55V no matter what conductor you touch, and that's a lot safer than the possibility of the full 110V and certainly a lot safer than the full 240V. Perhaps the chance of getting a shock is increased, but at least if it happens there's no way it will kill you. Also, unlike the blue transformer, an RCD on the 110V side of the transformer will do something, but these site transformers tend not to be fitted with them.

So why bother connecting the centre tap to mains earth, if all you're interested in is reducing the likely shock voltage to 55V? The reason is that if you're drilling a hole in a wall on a building site and hit a buried cable at 240V (or even 415V), you're going to want that fault current to go through your drill bit, through the casing of the drill and down the 110V earth wire and through the transformer to the 240V incoming side, to earth. Not through the drill bit, through your hand, heart, other hand, aluminium step ladder, muddy puddle, earth. Or maybe you're sawing some floorboards.

Someone also mentioned the prospective fault current is reduced with these yellow transformers. This is true, because the voltage is lower, assuming a constant earth impedance. But it has the unfortunate side effect of not tripping the output fuse or MCB as quickly. But that's probably irrelevant.


Speaking of RCDs and death, years ago I was diagnosing a Marshall guitar amp with the customer standing over my shoulder and breathing down my neck. It has an earthed folded metal chassis and the IEC inlet (with uninsulated spade terminals on the inside) on one side. I turned round to the customer to give him the lowdown on what was wrong with his amp. My left hand was in contact with the amp chassis, and as I turned my right hand touched the live spade on the back of the IEC. In other words, I had the full 255V mains (our mains is high at night) across my heart. FUUUUUCK! I screamed loudly as I flew back a few feet. The 30mA RCD tripped and the workshop was plunged into darkness. I think the customer was more shaken up than I was, which is saying something. Had I of used the blue isolation transformer, I would have been absolutely fine. Accidents happen in this business, no matter how experienced or careful you are. (Send your repairs to That Crazy Norfolk Bloke, Unit 3......)

[Monkeh]

It DOES have a centre tapped earth - the transformer secondary is wired as 55-0-55, with the 0 connected to mains earth. So between output 'live' and earth there's 55V and between output 'neutral' and earth there's another 55V.

They're not all earthed on the centre tap. Proper transformers for distribution on a site (rather than individual power tool supplies, which would require running 240V around the site.. kinda defeating the purpose) are, though.

So why bother connecting the centre tap to mains earth, if all you're interested in is reducing the likely shock voltage to 55V? The reason is that if you're drilling a hole in a wall on a building site and hit a buried cable at 240V (or even 415V), you're going to want that fault current to go through your drill bit, through the casing of the drill and down the 110V earth wire and through the transformer to the 240V incoming side, to earth. Not through the drill bit, through your hand, heart, other hand, aluminium step ladder, muddy puddle, earth. Or maybe you're sawing some floorboards.

The majority of tools these days are double insulated anyway.

Someone also mentioned the prospective fault current is reduced with these yellow transformers. This is true, because the voltage is lower, assuming a constant earth impedance. But it has the unfortunate side effect of not tripping the output fuse or MCB as quickly. But that's probably irrelevant.

True, but a short duration relatively low current fault is a hell of a lot less dangerous than a giant bang, especially when you've got people moving heavy objects and operating power tools. I have experience in just how big a bang a domestic supply can produce, and I'd rather it not have made me jump like that.

Speaking of RCDs and death, years ago I was diagnosing a Marshall guitar amp with the customer standing over my shoulder and breathing down my neck. It has an earthed folded metal chassis and the IEC inlet (with uninsulated spade terminals on the inside) on one side. I turned round to the customer to give him the lowdown on what was wrong with his amp. My left hand was in contact with the amp chassis, and as I turned my right hand touched the live spade on the back of the IEC. In other words, I had the full 255V mains (our mains is high at night) across my heart. FUUUUUCK! I screamed loudly as I flew back a few feet. The 30mA RCD tripped and the workshop was plunged into darkness. I think the customer was more shaken up than I was, which is saying something. Had I of used the blue isolation transformer, I would have been absolutely fine. Accidents happen in this business, no matter how experienced or careful you are. (Send your repairs to That Crazy Norfolk Bloke, Unit 3......)

My point exactly. The RCD reduced the duration of your shock, but did not prevent it or reduce the magnitude. RCDs are important, but so are isolation transformers. Neither should allow one to become complacent.

[saturation]

I use the second one, its 50% cheaper if you buy the variac separately and don't need the power rating of the Tenma variable AC.  I tore it down anyway to check build and tested it for leakage just to be sure.  The analog meters often are very inaccurate so not worth paying for it, I monitor the output with a DMM.

I bought this cheapo variac to provide variable AC:

http://www.mpja.com/prodinfo.asp?number=15162+TR

I am looking for a bench isolation transformer.

I thought about modifying a commercial one as described by Toddfun or purchasing?
http://www.mcmelectronics.com/product/72-1097#description
http://www.mcmelectronics.com/product/72-6670

http://www.phcenterprise.com/isolation.html


Any suggestions?

 
 

[lewis]

They're not all earthed on the centre tap. Proper transformers for distribution on a site (rather than individual power tool supplies, which would require running 240V around the site.. kinda defeating the purpose) are, though.

All site transformers should be centre tapped earth on the secondary. If you see one that isn't, throw it in the bin. BS7375:2010 specifies the use of a Reduced Low Voltage (RLV) system for distribution on construction sites, RLV being defined in BS7671:2008 as "A system in which the nominal line to line voltage does not exceed 110V and the nominal line to earth voltage does not exceed 63.5V". In other words, CTE.

The majority of tools these days are double insulated anyway.

Not all!

[Monkeh]

They're not all earthed on the centre tap. Proper transformers for distribution on a site (rather than individual power tool supplies, which would require running 240V around the site.. kinda defeating the purpose) are, though.

All site transformers should be centre tapped earth on the secondary. If you see one that isn't, throw it in the bin. BS7375:2010 specifies the use of a Reduced Low Voltage (RLV) system for distribution on construction sites, RLV being defined in BS7671:2008 as "A system in which the nominal line to line voltage does not exceed 110V and the nominal line to earth voltage does not exceed 63.5V". In other words, CTE.

Interesting, although the transformers are to comply with BS3535, not BS7375. You are correct in the requirement for CTE, however. Good luck convincing people to throw away their transformer, though.. I couldn't even get one to let me slide a cable with broken insulation back into the stuffing gland.

I'm going to assume you're familiar with working with site electrics and the hassles of getting self-employed tradesmen to care for their electrical tools properly. Case in point: Plasterers. *shudder*

[lewis]

I'm going to assume you're familiar with working with site electrics and the hassles of getting self-employed tradesmen to care for their electrical tools properly. Case in point: Plasterers. *shudder*

Ha! One of the plasterers I worked with would cut off your T+E right at the grommet if you left them protruding even a millimetre out of a flush mount pattress, or if they caught his trowel. He did it to me once on a spur at the bottom of a stairwell. So I chased 20ft all the way from the socket right up to the ceiling on the second storey landing to put a new bit of conduit in because I sure as hell wasn't taking up the floor. He had to reskim the whole wall. He didn't do it again.

Bastards!