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Old 15th Jul 2019, 5:21 pm   #21
ajgriff
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Default Re: Isolation/isolating transformers

Lawrence might be right but I don't think that invalidates the point the author is emphasising in the relevant section of the article.

Alan
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Old 15th Jul 2019, 5:47 pm   #22
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Default Re: Isolation/isolating transformers

My view on this is that any piece of safety equipment can make things more dangerous if you assume it will always get you out of trouble. But used intellegently of course it can make things safer.

One side of the mains supply is eathed at some point. So if you (earthed) touch the other side you will complete the circuit (mains through you to earth, then back to the other side of the mains). Zap!. Similarly if you connect an earthed test instrument ('scope etc) to a piece of circuity that is directly connected to the mains (like a live chassis radio) you can end up shorting things out via the earth connections. Not a good idea.

The output of the sort of isolating transformer we are discussing is not connected to earth at any point. That is the reason for it. So now if you touch one point you will not get a shock, or at least not a dangerous ones (there's always some leakage I guess). You can connect your 'scope to the device without shorting anything. But note that if you do that, you've now earthed some point in the circuit meaning that if you then touch another point you might get a dangerous shock).

My feeling is that an isolating transformer is of little use on things with a normal mains transformer anyway. It's useful on live-chassis radios and TVs. It's essential if you are working on switch-mode power supplies.
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Old 15th Jul 2019, 6:25 pm   #23
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Default Re: Isolation/isolating transformers

Quote:
Originally Posted by TonyDuell View Post
... One side of the mains supply is earthed at some point. So if you (earthed) touch the other side you will complete the circuit (mains through you to earth, then back to the other side of the mains). Zap!
But if all works as it should the RCD in your (modern) consumer unit will trip and save you from dying.

Quote:
The output of the sort of isolating transformer we are discussing is not connected to earth at any point ... [so] you can connect your 'scope to the device without shorting anything. But note that if you do that, you've now earthed some point in the circuit meaning that if you then touch another point you might get a dangerous shock.
And now the shock current won't generate a live-neutral imbalance back at the consumer unit. So by adding an isolating transformer, which you thought would make you safer, you've actually disabled the thing which previously protected you. Let's hope your heart can cope without it ... or that you've also got some sort of earth leakage detector in the supply.

Cheers,

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Old 15th Jul 2019, 6:38 pm   #24
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Default Re: Isolation/isolating transformers

Yes the RCD _should_ trip if you connect yourself to a live point. But you know as well as I do that electronic devices can fail. And while I am glad I have an RCD (It's one more safety device that might help) I'd rather not depend on it. I will still take care not to complete a circuit through me.

How many people actually test their consumer unit RCD as suggested? I actually do (press the test button which causes an imbalance in the currents when I get the quarterly electricity bill through the door). But most people (probably not the people on this forum) have never heard of doing this.

As for an earth leakage detector on the output of an isolating transformer, that rather defeats the purpose of such a transformer as it would trip as soon as I connected my 'scope or whatever. Perhaps a device that somehow detects currents through me.

Or perhaps I will just use the isolating transformer so I can connect the 'scope to the chopper circuit in an SMPSU but carry on working as if everything is live (which indeed it could be). One hand in my pocket, turn off and discharge whenever possible before touching anything, etc.
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Old 15th Jul 2019, 9:33 pm   #25
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Default Re: Isolation/isolating transformers

Wow this has stirred up a lot more debate than I though it would. I will keep an open mind for now and proceed with my usual caution. Anyone can have a brain fart, no matter how experienced; I think anything which can add a layer of safety in a moment of stupidity/forgetfulness etc. is welcome.
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Old 15th Jul 2019, 11:32 pm   #26
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Default Re: Isolation/isolating transformers

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Originally Posted by jhockridge View Post
Good afternoon. Could anyone help me source a 400-500w isolating transformer ? I think it would be a really sensible idea to plug my projects into when aligning/troubleshooting. I was also wondering if a variac is a good plan as they used to use them a lot in the lab I worked in.

Thanks for helping me out. James
Wickes have site trannies for £68. https://www.wickes.co.uk/Defender-3k...Tr…/p/146209 A pair back to back gives great isolation and 3kVA for £136. Bargain!
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Old 16th Jul 2019, 6:25 am   #27
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Default Re: Isolation/isolating transformers

Thanks for that clarification.

Several iso tfmr's available on ebay - https://www.ebay.co.uk/sch/i.html?_f...ormer&_sacat=0 though not many 240 240 single phase types.

Andy.
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Old 17th Jul 2019, 12:02 pm   #28
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Default Re: Isolation/isolating transformers

Quote:
Wickes have site trannies for £68. https://www.wickes.co.uk/Defender-3k...Tr…/p/146209 A pair back to back gives great isolation and 3kVA for £136. Bargain!
Be aware (of course, most members here will not need this reminder) that 110V site transformers are actually 55-0-55V with the centre tap earthed and connected to the earth connector on the output socket. Therefore using two of these back to back will not provide isolation unless the earth is lifted somewhere in the chain.
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Old 17th Jul 2019, 12:07 pm   #29
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Default Re: Isolation/isolating transformers

It's true that the intermediate 55-0-55 windings won't be isolated, unless as you say their earth connections are lifted, but the 0-240V winding on the second transformer will be isolated won't it ?

Cheers,

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Old 17th Jul 2019, 12:30 pm   #30
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Default Re: Isolation/isolating transformers

Certainly, but depends how the 240 output is treated. (Thinks thoughtless 3 wire connection to a 13A socket. "Surely it will be safer if you include an earth ....")
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Old 17th Jul 2019, 1:53 pm   #31
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Default Re: Isolation/isolating transformers

Quote:
Originally Posted by GroovyG View Post
Certainly, but depends how the 240 output is treated. (Thinks thoughtless 3 wire connection to a 13A socket. "Surely it will be safer if you include an earth ....")
The 240v winding now used as the output won't be referenced to earth in any way, so still isolated.

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Old 17th Jul 2019, 4:50 pm   #32
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Default Re: Isolation/isolating transformers

Rather than mess about with 2 transformers 'back to back' why not just buy a 230V-230V isolating transformer, for example :

https://uk.rs-online.com/web/p/safet...rmers/1226713/

It's 1.5kVA which should be enough for any vintage radio, and it's not much more expensive than a pair of the site transformers menitoned earlier.
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Old 17th Jul 2019, 11:27 pm   #33
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Default Re: Isolation/isolating transformers

Indeed, two el cheapo site transformers is a DIY solution. It would involve plenty of work, but might be good thing for a workshop. You could have a permanent, isolated, 110V outlet on your bench as well as an isolated 230V outlet.

I'm not saying it's the best way, it's just A way to get an isolated supply.
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Old 17th Jul 2019, 11:38 pm   #34
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Default Re: Isolation/isolating transformers

The cheapest 230:230 transformer that RS do is £125. https://uk.rs-online.com/web/p/safet...rmers/1226717/ It's 750VA.
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Old 18th Jul 2019, 5:11 am   #35
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Default Re: Isolation/isolating transformers

Quote:
Originally Posted by Brian of Romsey View Post
Indeed, two el cheapo site transformers is a DIY solution. It would involve plenty of work, but might be good thing for a workshop. You could have a permanent, isolated, 110V outlet on your bench as well as an isolated 230V outlet.

I'm not saying it's the best way, it's just A way to get an isolated supply.
Remember that the output of a 110V site transformer is not isolated from earth, the centre tap is earthed. So such a transformer will not give an isolated supply unless you modify it. Often the transformer is put in the case in potting compound and there is just one green/yellow wire coming out of said potted part which is the centre tap of the secondary winding and I assume the core etc. Whether you are happy having the core not earthed (but connected to the centre tap of your isolated supply) is up to you.

You would also have to modify the connector arrangements, either by wiring the 110V output to a BS4343 plug (sorry, I forget what the CEN number is) or by making a (IMHO dangerous) cable with such a plug on each end to link the 2 transformers. And then fit a BS1363 13A socket on the 230V cable of one of the transformers.

If you have the site transformers hanging around you might as well try it If not, by the time you've bought a pair of them and the connectors and spent time modifying them it's probably better to buy the 230V-230V transformer. I'm all for DIY solutions (of course), but...
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Old 18th Jul 2019, 5:15 am   #36
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Default Re: Isolation/isolating transformers

The Isolating transformer only somes adds a safety factor...only when it supplies one isolated appliance and no other appliances are powered by it at the same time, so say a non double insulated drill outside in a garden. Why ? (Shouldn't need explaining) this...because simply if you have more than one appliance connected, one of them can , with a fault, ground one of its supply feeds with respect to ground and you are back to square one with one live supply above ground.

However, in the workshop application, where you are connecting different appliances to each other, the role of the transformer is completely different (not safety at all). It allows you to isolate a hot chassis set, so you can ground its chassis and connect test equipment like generators and scopes to it, without tripping you RCD, because, as I have mentioned many times before the isolation transformer defeats the dwelling's RCD.
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Old 18th Jul 2019, 7:17 am   #37
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Default Re: Isolation/isolating transformers

One advantage of a small isolation transformer, that often gets overlooked, is that the PSCC ( prospective short circuit current) will be significantly reduced. This means that a fault which causes a large current to flow in the equipment, will not cause so much damage to main switches, etc.

I’m sure most of us have seen or witnessed a glass mains fuse rupture due to a short in a piece of equipment.

As with all electrical equipment, Isolation transformers have their uses, but it’s how we use them that’s important. As has previously been stated, it’s not a magic box to prevent us getting a shock, just an aid if used intelligently.


Cheers. SimonT.
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Old 18th Jul 2019, 8:56 am   #38
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Default Re: Isolation/isolating transformers

Hi All

My two'pennth....

During my career, such as it was, I often told off junior engineers for using 'Wizard Words' when trying to understand what was going on in a circuit. These are such terms as 'earth', 0V, floating and 'return'. The point being that they only represent anything useful if you fully understand the context that they are being used in - or you are a Wizard.

My second point was that electrons can't read and just go where they are pushed, so don't imagine that they will go down the '0V' line just because you wrote that on the circuit diagram.

So - don't speak it, english is too flexible - use a circuit diagram and the laws of physics to understand why things may or may not be safe.

Earth - Really, has anyone got both their bare feet buried in soil? Planet Earth has electrostatic associations and Mains Earth has 50Hz current aspects - which is being considered? (Dumb idea to call Mains Earth, Earth)

Floating - That depends on the frequencies, charge and capacitance of the associated bodies surely? Again, what is the context?

It took me a while to understand why connecting a Neutral to Mains Earth would cause the MCB to trip or why Neutral was a variable potential above the house Mains Earth. But a circuit clears that up nicely.

So, I suggest draw out the 'circuit' and be those electrons. Don't assume that any point in the circuit is fixed or known. The potential of interest is the one between all possible human points of contact. Avoid terms that make you assume, so label each circuit node A, B, C, etc..

Don't assume that you have to have a current path 'across the heart' either, the human body is a network of resistances - apply a voltage across any two points of such a network and potentials will appear everywhere. Again a circuit analysis shows what the electrons will do and not what humans assume.


Cheers
James
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Old 18th Jul 2019, 10:34 am   #39
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Default Re: Isolation/isolating transformers

I laugh when folks say an isolating transformer was not used for safety reasons.....certainly was where I worked.

Lawrence.
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Old 18th Jul 2019, 11:14 am   #40
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Default Re: Isolation/isolating transformers

If you connect a live chassis set directly to the mains (as you would in normal use) then you will get an electric shock if you touch the part connected to mains live and something connected to mains earth (water pipes, test gear, etc) simultaneously. To minimise the shock risk, you should ensure the mains neutral is connected to chassis, and mains live feeds the HT, you’re less likely to touch HT than the chassis. If you have an RCD somewhere in your mains supply, then the duration of any electric shock will be very short, hopefully saving your life.

If you connect this same live chassis set to an isolation transformer (with no earth connection) it doesn’t matter which side is connected to chassis, you will not get a shock if you touch mains earth and anywhere in the set simultaneously.

If in either scanario, you touch the chassis of the set and the HT simultaneously, you will get a shock, the transformer will not save you, but neither will the RCD, because the current flow is from live to neutral, not live to earth, so there is no unbalanced current in the RCD.

An RCD will limit the duration of an electric shock in some circumstances where in the same circumstances an isolation transformer will prevent the shock occurring at all. In other circumstances, neither will save you from a shock.

The other big advantage of an isolation transformer is that it allows you to connect earthed test gear to a live chassis set.

Stuart
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