UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Other Vintage Household Electrical or Electromechanical Items

Notices

Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

Reply
 
Thread Tools
Old 27th Jan 2023, 9:35 am   #41
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,436
Default Re: Isolator Transformer Query

There are lots of different applications for isolating transformers, and in some of them it's necessary to earth one side of the secondary, some it's useful to earth a centre tap on the secondary, and in others it's absolutely essential *not* to earth any part of the secondary.

There's no "one size fits all" best way to do it. The only way to determine how the earthing should be done is to have a clear understanding of what the isolating transformer is to achieve, and the environment and electrical safety regime it's to operate in. It's impossible, and indeed potentially dangerous, to make a blanket statement of "this earthing arrangement is best".

Chris
__________________
What's going on in the workshop? http://martin-jones.com/
cmjones01 is offline   Reply With Quote
Old 27th Jan 2023, 10:16 am   #42
GMB
Nonode
 
GMB's Avatar
 
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 2,931
Default Re: Isolator Transformer Query

Absolutely right!

I think some confusion originates from the word "isolating" in the name, the usage varies depending on what aspect you are trying to isolate.
GMB is offline   Reply With Quote
Old 27th Jan 2023, 10:44 am   #43
G6fylneil
Pentode
 
Join Date: Sep 2011
Location: Coventry, West Midlands, UK.
Posts: 119
Default Re: Isolator Transformer Query

Seconded!
G6fylneil is offline   Reply With Quote
Old 27th Jan 2023, 12:04 pm   #44
stuarth
Heptode
 
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 542
Default Re: Isolator Transformer Query

For the case of the "testing machine" described above, isn't the transformer part of the installation? For transformers for specific applications, there are many ways you might want to connect an earth, and hopefully the designer of such equipment understands the issues and designs accordingly.

The discussion here is really about a general purpose transformer on the bench to provide an isolated mains supply, originally for safety on live chassis vintage radio and TV sets, but also for a variety of more modern equipment, often using a SMPS, where we need a fully floating supply, not nessessarily for safety, but to allow unrestricted connection of test equipment anywhere in the circuit. In this case, the transformer should have no fixed earthing on the secondary.

On a closely related subject, the same argument could/should be applied to the earth pin of the output socket on the general purpose isolating transformer, it should not have a fixed earth either. This is to guard against the very dangerous crossed N-E connection mentioned on this forum many times.

I would say the blanket rule might be something along these lines:-

For a transformer in a specific application, the designer should connect an earth to an appropriate place, (eg a point on the transformer secondary, or the earth pin of the transformer output socket to absorb leakage currents from filters).

For a general purpose mains isolation transformer, do not have any fixed earth connection, either to the secondary, or to the output socket on the transformer. The user is then free to connect an earth anywhere at the device under test if required.

Note that an earth does not always improve safety - see the drawing of the N-E crossed connection earlier, or just ask the sparrows on the power line.

Stuart
stuarth is online now   Reply With Quote
Old 27th Jan 2023, 12:18 pm   #45
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 4,738
Default Re: Isolator Transformer Query

Quote:
Originally Posted by stuarth View Post
For the case of the "testing machine" described above, isn't the transformer part of the installation? For transformers for specific applications, there are many ways you might want to connect an earth, and hopefully the designer of such equipment understands the issues and designs accordingly.
Agreed. After all the mains transformer in a valve audio amplifier is an isolating tranformer and it is common to earth the centre-tap of the HT secondary winding.

Quote:
Originally Posted by stuarth View Post

On a closely related subject, the same argument could/should be applied to the earth pin of the output socket on the general purpose isolating transformer, it should not have a fixed earth either. This is to guard against the very dangerous crossed N-E connection mentioned on this forum many times.
I've seen plenty of mains leads with L-N swaps, but never E swapped with anything else. It can happen, I guess, but it is not common. Also, I would recommend checking (visually and electrically) the wiring of anything before you power it up, even if you are going to use an isolating transformer. I always make sure the earth wire connects to what I expect, for example

If you have a mains filter with the traditional delta-connected capacitors in the device under test and are connecting an earth to some point in the 'mains' circuit of that device (e.g. by connecting a 'scope to it) then I feel it's a good idea to have the earth pin it connected to true earth. Otherwise you might get enough current flowing through said capacitors to give you a tingle, and more importantly enough current to do damage to circuitry on the output side if some point there happens to end up earthed.

Yes you can use a clip lead on the chassis to do this. But I find it easier to use the earth wire of the mains cable, it won't fall off at the wrong moment and touch something else.

My homebrew isolating transformer has a pair of 4mm sockets on the front of the box. One is connected to mains earth (via the input mains cable).The other is connected to the earth pin of the output socket. There's a link I can insert or remove as required.
TonyDuell is offline   Reply With Quote
Old 27th Jan 2023, 4:17 pm   #46
stuarth
Heptode
 
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 542
Default Re: Isolator Transformer Query

I agree that a N-E swap is likely to be very rare (I believe the problem was highlighted by the BBC for gigging musicians bringing their own amps etc into the studio), but it could happen, and is a very dangerous problem because it could give two metal boxes in close proximity (the device under test and a piece of test gear, ie two things you are very likely to touch simultaneously) with 240V between them.

With a standard UK 13A socket for the transformer output, it's actually a bit awkward to remove the earth connection, you generally have to cut through the metal strap used to connect the earth pin to the mounting screws.

Like many modern safety features, it's a trade off between the cost of the safety feature (cutting an earth strap and having to use a separate earth wire sometimes) the risk of a dangerous fault occuring (N-E swaps are likely to be rare for most of us), and the consequences of not having the feature (death in the worst case).

Stuart
stuarth is online now   Reply With Quote
Old 27th Jan 2023, 4:46 pm   #47
Lucien Nunes
Nonode
 
Lucien Nunes's Avatar
 
Join Date: Apr 2008
Location: London, UK.
Posts: 2,364
Default Re: Isolator Transformer Query

N/E swap is probably much less common now that many socket-outlets have RCD protection. It cannot go undetected in use as it could before RCDs. It used to be possible for an appliance to work with any of the three mains lead cores connected to neutral, or with all three in the wrong terminals.
__________________
Three anodes good, six anodes better!
Lucien Nunes is offline   Reply With Quote
Old 27th Jan 2023, 5:35 pm   #48
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 4,738
Default Re: Isolator Transformer Query

I feel that the 'There is no one right answer' that has been previously suggested in this thread certainly applies.

If you are using an isolating transformer on audio equipment and you don't inspect/check the wiring of the equipment before plugging it in, as I believe the BBC were doing, then it makes sense not to have the earth pin connected. There is a slim chance of an undetected fault that could have fatal results

If on the other hand you are using the isolating transformer on an electronics workbench for testing/repairing mains-connected electronic circuitry (AC/DC radios and TVs, switch mode power supplies, etc) then there are times when having that earth pin solidly earthed is a good idea. I feel that anyone who knows to use an isolating transformer in such cases also knows how to inspect and test the mains wiring and will do so before plugging anything in.
TonyDuell is offline   Reply With Quote
Old 28th Jan 2023, 8:34 am   #49
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Default Re: Isolator Transformer Query

The real problem is, there are two quite distinct reasons why you might want a piece of equipment to be supplied by an isolating transformer.

Scenario 1: You want to avoid creating a dangerous situation if any exposed metalwork on the appliance should come into contact with one side of the power supply. This might happen using an electric shaver in a bathroom, or an unknown guitar amplifier in a TV studio, for instance. In this case, the secondary needs not to be Earthed, and appropriate care should be taken to minimise parasitic coupling to Earth: keep all connections short and don't have any "real" Earth connection to the appliance (even the capacitance between adjacent conductors in a mains lead may let enough current flow to be noticed as a "tingle"). The deliberately-introduced, very high impedance between the appliance supply and Earth will limit the current in case any fault brings the user (assumed to have a low-impedance Earth connection) into contact with either terminal of the appliance supply.

Scenario 2: You want a "clean" Earth reference for test equipment connected to a live-chassis appliance, excluding the building wiring back to wherever the Neutral and Earth are joined (usually right next to the company head, in an urban area). In this scenario, one end of the secondary is Earthed, and so is the test equipment (oscilloscope, signal generator, Arduino connected to PC, or similar). Any safety advantage of a floating secondary is negated by the Earth connection, but it means the appliance's chassis is connected directly to a common Earth with the test equipment; and specifically, not in series with some unknown impedance which is also carrying a varying current to other, unrelated appliances in the building.

The real danger is thinking that an isolating transformer is magic. Just because it can sometimes offer protection from a dangerous situation, doesn't mean it can't be used in a way that won't offer that protection.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments.
julie_m is offline   Reply With Quote
Old 28th Jan 2023, 10:47 am   #50
Lucien Nunes
Nonode
 
Lucien Nunes's Avatar
 
Join Date: Apr 2008
Location: London, UK.
Posts: 2,364
Default Re: Isolator Transformer Query

The two scenarios identified by @julie_m are well explained and the final statement is a very good one-line summary. However, in my view, neither of those two scenarios encompass all the main reasons that I would plug an electronic device into my bench isolating transformer. Two common ones would be:

* To reduce the risk of shock to earth via casual contact with live functional parts that have been made accessible for maintenance purposes. I.e. the parts are intended to be live in operation but not intended to be exposed to the touch.

* To enable test equipment to be connected to the unit in such a way as neither conductor of the supply cable would be at or near earth potential. For example, earthing the negative of full-wave rectified mains.

In both of these cases, the secondary must be isolated. In @julie_m's scenario 2, the other bench-based use-case, identical potentials to earth will exist on the various parts once the test-gear earth is taken to the DUT, regardless of whether the transformer secondary is earthed. Therefore, somewhat contrasting with the picture painted by that description, the 'default' situation for a service-bench isolating transformer is for a floating secondary.
__________________
Three anodes good, six anodes better!
Lucien Nunes is offline   Reply With Quote
Old 28th Jan 2023, 10:49 am   #51
stuarth
Heptode
 
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 542
Default Re: Isolator Transformer Query

In the music studio scenario, eg the BBC, you could have a band come into the studio with various pieces of equipment in assorted sizes and a box of mains leads and extension cables. You would have to check every piece of equipment, and both ends of every mains cable. If you miss one, including that extra cable someone nipped back to the van for, someone could die.

So you take the safe option and remove the earth at the transformer output. If there are any N-E swaps, the equipment will not power up, and it's all fail-safe.

Effectively, the isolation transformers in the studio have only one job, and never need an earth connection (no hum loops either!). For a wider range of applications, adding two 4mm sockets to link the output socket earth to mains earth if required is useful (wouldn't do in the BBC case, someone in the band could re-connect the earth when trying to get their equipment to power up - horses for courses).

Stuart
stuarth is online now   Reply With Quote
Old 28th Jan 2023, 4:35 pm   #52
Roger Ramjet
Hexode
 
Join Date: Feb 2022
Location: Leicestershire, UK.
Posts: 382
Default Re: Isolator Transformer Query

In the 70's the company I worked for imported several USA made yarn spinning machines, each with multiple 3 x phase motors & dedicated control gear.

The control contactor worked on 24VAC & to facilitate this a 415V > 24V step down transformer was installed, the secondary winding of which was floating.

After a while, several transformer started to "break down" resulting in a phase voltage of 415 appearing on the control circuit. Each one was replaced with a UK match but as a precaution I tied one leg of the 24V secondary down to the machine earth & then retrospectively modified the remaining USA made ones.

Made me sleep easier at night.

Rog
Roger Ramjet is offline   Reply With Quote
Old 28th Jan 2023, 5:04 pm   #53
Lucien Nunes
Nonode
 
Lucien Nunes's Avatar
 
Join Date: Apr 2008
Location: London, UK.
Posts: 2,364
Default Re: Isolator Transformer Query

Inter-winding breakdown of that kind ought to be vanishingly rare in a panel transformer. It's a reminder that when using a transformer to provide an electrically separated supply, it is not really sufficient simply to know that the primary and secondary can withstand an insulation test between them. The whole thing should be made to a recognised standard, e.g. BS3535, which will impose minimum standards for many parameters of the insulation.

This thread is mainly about 230-230V transformers for bench use in maintenance work, but the requirements for isolation are common to ELV applications too. In @Roger Ramjet's machine, the ELV side appears to have been what we would now consider SELV, so the transformer would need to be made as a Safety Isolating Transformer to the above standard. By converting the output to PELV, some of the requirements are relaxed.

Did you find the root cause of the insulation breakdown? Some USA-made transformers run very much on the border of saturation on 50Hz and if the voltage is also high (e.g. when 208V equipment is supplied from a 230V stepdown) might get undesirably hot.
__________________
Three anodes good, six anodes better!
Lucien Nunes is offline   Reply With Quote
Old 28th Jan 2023, 7:07 pm   #54
Roger Ramjet
Hexode
 
Join Date: Feb 2022
Location: Leicestershire, UK.
Posts: 382
Default Re: Isolator Transformer Query

Quote:
Originally Posted by Lucien Nunes View Post

Did you find the root cause of the insulation breakdown? Some USA-made transformers run very much on the border of saturation on 50Hz and if the voltage is also high (e.g. when 208V equipment is supplied from a 230V stepdown) might get undesirably hot.
Hi Lucien, No I never investigated the cause & just assumed they were a bad batch because percentage wise, only a few failed.

I take you point though that may not have liked the UK frequency or voltage.

Rog
Roger Ramjet is offline   Reply With Quote
Old 29th Jan 2023, 4:55 pm   #55
kellymarie
Pentode
 
Join Date: Sep 2020
Location: Weymouth, Dorset, UK.
Posts: 102
Default Re: Isolator Transformer Query

When I got my little isolating transformer someone told me I should earth one leg of the secondary this never seemed right to me so I didn't do it I asked my local TV repair shop how there's were wired the boss told me on no account earth the output so I never have and its been fine. That's my experience in the real world
kellymarie is online now   Reply With Quote
Old 29th Jan 2023, 7:57 pm   #56
Roger Ramjet
Hexode
 
Join Date: Feb 2022
Location: Leicestershire, UK.
Posts: 382
Default Re: Isolator Transformer Query

On a grander scale, I am sure I recall our lecturer at Tech saying that the grid network transformers always had the the Neutral star point of the secondary earthed to prevent the very high transmission voltages getting onto the local network in the event of a winding breakdown.

Rog
Roger Ramjet is offline   Reply With Quote
Old 29th Jan 2023, 8:17 pm   #57
kellymarie
Pentode
 
Join Date: Sep 2020
Location: Weymouth, Dorset, UK.
Posts: 102
Default Re: Isolator Transformer Query

Also all distribuiton transformers have the star point solidly earthed to avoid capacitive coupling raising the LV side above the correct 250/433 volts
kellymarie is online now   Reply With Quote
Old 30th Jan 2023, 5:10 pm   #58
Roger Ramjet
Hexode
 
Join Date: Feb 2022
Location: Leicestershire, UK.
Posts: 382
Default Re: Isolator Transformer Query

Quote:
Originally Posted by kellymarie View Post
Also all distribuiton transformers have the star point solidly earthed to avoid capacitive coupling raising the LV side above the correct 250/433 volts
That's very interesting Kellymarie & would mirror what was summed up from the OP symptoms.

Noting the size & primary voltage of the grid transformers, would that capacitive coupled voltage be [very] dangerous if not earthed ?

Rog
Roger Ramjet is offline   Reply With Quote
Old 30th Jan 2023, 5:25 pm   #59
kellymarie
Pentode
 
Join Date: Sep 2020
Location: Weymouth, Dorset, UK.
Posts: 102
Default Re: Isolator Transformer Query

Hi Roger Ramjet it's possible that the secondary voltage could rise to many thousands of volts it would rise until some insulation either in the transformer or on cables and switchgear broke down this would then lead to failure of the equipment another good reason to tie LV windings to earth very firmly
kellymarie is online now   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 4:04 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2023, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.