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 > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 22nd Feb 2017, 8:41 am   #41
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Using isolation transformers.

Fixing old radios at home isn't 'work'

But it's interesting to consider what precautions are usually put in place to satisfy the terms of the electricity at work act. Working on anything at all live is not supposed to be done unless it can be done no other way. Suitable precautions must be taken. Suitable safety equipment must be provided. Other people have to be around to render aid in case of accident, and there should be trained first-aiders. No voltage is considered safe under all circumstances simply because it is low. Most firms with people working in an indoors warm dry environment have declared the old SELV limit of 50v to be safe. RCDs are treated as additional safety devices and must not be relied on for primary safety.

This does sound rather draconian, and what on earth does 'suitable' mean? There's enough wiggle room there to keep thousands of lawyers fed, clothed and provided with golf club memberships for life. It's a recipe for assigning blame after an event, it's not really a recipe for preventing the event in the first place. It works by scaring employers into taking extensive precautions and having them all documented so that they can be pointed at defensively should anything go wrong.

There is a general principle in making things safe that at least two things would need to go wrong before anything becomes dangerous, and that there should be a warning should any of these protections fail.

Brakes are the oft-used example. Everything made for decades has had dual circuit brakes so the hydraulic systems are duplicated. But there is only one reservoir for fluid. Actually that reservoir is divided in two by an internal wall, so loss of fluid due to a leak in one system doesn't drain the other. The wall only goes part way up, and the fluid level is above it. There is a float switch monitoring the fluid above the wall. If one circuit leaks, the level drops and the warning light comes on to show that attention is needed. What if the bulb's failed and isn't there for when you need it? The same bulb is used as a handbrake warning light. So when you switch the car on, you should check all the warning lights appear. This little fact is buried in those owner's manuals that no-one ever reads. No one ever checks or bothers if one doesn't light. So most people keep on driving even if one circuit has lost its fluid. It only gets noticed at a service or when the remaining circuit fails.

So all the careful planning of back-up systems comes to almost nothing because of human factors.

For radio/tv fixing I always use an isolating transformer when working on anything with a live chassis or when working in a mains SMPS. I do not trust neutral to remain safe to touch. I attach a solid ground so that as a last ditch a short will pull a fuses or MCB. And then I'm very careful to keep hands away from nasty bits. I use a scope as my primary piece of test equipment, and I'm not prepared to mess around with some test gear in a stack 'floating' You soon don't know what's safe to touch and what isn't and instincts can be far wrong. One hand behind back, and one probe at a time.

Could I morally advise anyone else to take less precaution than I choose for myself?

No, I'm not a safety nutter. I ride 'difficult' horses! I've worked professionally developing prototype mains switch mode supplies of unusual architecture with up to kilowatts circulating. I've fixed valve transmitter equipment again with kilowatt and kilovolt league big transformer supplies. I built myself oscilloscopes with mains transformer powered EHT when I was a schoolboy. And in all those years I've had one shock. It was fixing my own TV in my own living room, on my own, on an isolating transformer and I brushed the PCB on the end of the CRT and got a zap at focus voltage. Ye gods it smarted.

The primary safety factor is human. It's how you work, how disciplined you are to always do some things and to never do others. And it's these behaviours that beginners haven't yet learned. Transformers or RCDs aren't a magic solution. They are a good idea for some extra protection.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 22nd Feb 2017, 9:02 am   #42
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Using isolation transformers.

Quote:
Originally Posted by stuarth View Post
I'm not sure I agree with your first point about always using an isolation transformer.

As Mark has pointed out, an RCD is safer than a transformer.
That might be debatable. I have come across a number of RCD's that don't trip. The mechanism of failure in the ones I dismantled was warping of the plastic housing due to chronic heating of the RCD body, jamming the trip mechanism. So they are definitely not 100% foolproof. I think this is why its recommended to check them from time to time.

Also, the RCD actually has to be fitted to the circuit feeding the GPO(general purpose outlet) and some dwellings still don't have them, but you can be sure the isolation transformer is there, right in front of you. But there is the option of a compact RCD at the GPO in the workshop.

Also of course, in the hot chassis set, assuming the plug is polarized correctly, the chassis is connected to neutral, so handling many parts of the circuit with respect to the chassis will shock the person & won't trip the RCD. Nor will an RCD be tripped by the secondary side of a power transformer's HV supplies in the usual transformer type of set. And nor will the RCD trip using the isolation transformer if the current path is between a part of the circuit and the chassis connection.

So one point that could be added to the list is: Just like isolation transformers, the RCD can provide a false sense of security with people thinking that they are protected by a "safety device" when the specific circumstances dictate they are not.

In addition without the isolation tranny, with the chassis body connected to neutral (which often has a few volts ripple compared to earth) connecting this to a scope's earth can cause an earth loop like effect upsetting measured signals, but at least that is not a great safety issue unless there is no RCD and the chassis is phase and it gets connected to earth by the scope clip.

The one good reason to use the isolation transformer is to ensure the chassis is never connected to mains phase due to plug polarization issues and it removes that risk, RCD present at the power box or not.

I seem to agree with everything radiowrangler said in the post above too.

Last edited by Argus25; 22nd Feb 2017 at 9:09 am.
Argus25 is offline  
Old 22nd Feb 2017, 9:32 am   #43
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
Default Re: Using isolation transformers.

A worked in a dark room (some say I should still) with a high voltage rail overhead for EHT supplies to the tubes on test. We had Brandenburg 100kv power supplies, Tek scopes with cameras. test signal generators etc in there.
Rubber floor and bench, stool with insulators on the feet. Pull cord light switches and a big red plastic kill button.
I have sat there taking film shots on scopes with everything including me at 30kv above earth in total safety because nothing was earthed.
Every piece of equipment was powered in a 1 gang socket fed by its own isolation transformer, all contactable surfaces were insulated.
Compare this to the average cellar/shed/attic and the need for good isolation becomes more apparent.
Boater Sam is offline  
Old 22nd Feb 2017, 9:58 am   #44
stuarth
Heptode
 
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 665
Default Re: Using isolation transformers.

I wouldn't argue at all over the usefulness of an isolation transformer - I have on my bench a box with an isolation transformer, Variac, output meters, and a switched 13A socket which is the default mains supply for equipment under test. It's a big heavy box, but being on the bench it's very easy to unplug the equipment under test at appropriate times, I can increase the voltage slowly while watching the supply current, and I can connect earthed test gear when required. And I know where the high voltages are and can (usually!) avoid touching them.

A newcomer may not (yet) have an isolating transformer, but with an RCD protected supply, a multimeter, and enough knowledge (or guidance) to check that the chassis is at mains neutral, then there is some protection, even against touching the HT rails (which you don't get if there is a transformer in the path)

Of course either the RCD or the isolation transformer could be faulty ( Skywave reported a non isolating isolation transformer), but the RCD is easier to test, it has a built in self test feature.

Stuart
stuarth is offline  
Old 22nd Feb 2017, 10:49 am   #45
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Using isolation transformers.

Connecting up an isolating transformer, how hard can it be...just remember the word isolation.

Lawrence.

Last edited by ms660; 22nd Feb 2017 at 10:59 am.
ms660 is offline  
Old 22nd Feb 2017, 12:21 pm   #46
G4_Pete
Hexode
 
Join Date: Feb 2014
Location: Southampton, Hampshire, UK.
Posts: 419
Default Re: Using isolation transformers.

It is probably important to highlight the difference between a Tool transformer and a galvanic isolated transformer ( Even galvanically isolated are again sub divided into insulation clases) Both are marketed under the general term isolation transformer.

So for someone like me who was looking to make an informed choice on purchasing an approved isolation transformer , rather than a back to back variety I use there are still plenty of variables to consider.

RS Data sheet attached showing earth connected to one side of the isolated output.
Attached Thumbnails
Click image for larger version

Name:	tool transformers.jpg
Views:	158
Size:	32.2 KB
ID:	138026  
G4_Pete is offline  
Old 22nd Feb 2017, 1:11 pm   #47
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
Default Re: Using isolation transformers.

That's a tool transformer designed to operate double insulated tools with 2 core cables that have no earth connection and as you say it is not an isolation transformer.
Cardinal rule is that there should be no earth connected to anything other than the input.
Boater Sam is offline  
Old 23rd Feb 2017, 1:10 am   #48
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Using isolation transformers.

Quote:
Originally Posted by Boater Sam View Post
Cardinal rule is that there should be no earth connected to anything other than the input.
I don't think matters are quite as clear cut as that. It is apparent that there is some confusion over what, exactly, constitutes an 'isolation transformer'. And that confusion seems to be centred on what, exactly, is 'isolated'. If there is no d.c. connection between incoming L and N and the outgoing "L" and "N", then that could be called an 'isolation transformer': the incoming L and N are 'isolated' from the outgoing "L" and "N". OTOH, if the earth is also fed through, then that earth isn't 'isolated', so on that basis, it isn't an 'isolation transformer'. And then there are other configs. to obfuscate matters even further. So how do we resolve that confusion?

For myself, the only firm conclusion I can reach is to draw the obvious and distinct line between an 'auto transformer' on the one hand and an 'isolation transformer' on the other, where in the latter case, there is no d.c. connection between pri. and sec. and with no earth connections involved at all - except only for the frame of that transformer. With that type of transformer, arranged as an assembly, with the wiring configured to, from and around it, any of the previously-mentioned configs. can be met - but that transformer, per se, still remains an 'isolating' type: pri. and. sec. are not connected, d.c.-wise, plus the supply earth connected to the transformer's frame - and that could be considered as optional - but I wouldn't recommend it.

So ultimately, it seems that it is up to the user / installer of such a transformer to configure his / her 'isol. transformer assembly' according to his identified needs. And that was effectively dealt with by an earlier post by Mark Hennessey, q.v.

Al.

Last edited by Skywave; 23rd Feb 2017 at 1:16 am.
Skywave is offline  
Old 23rd Feb 2017, 5:51 pm   #49
dave cox
Nonode
 
dave cox's Avatar
 
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,059
Default Re: Using isolation transformers.

I think radio wrangler spelled it out, the only sensible reason for using an isolation transformer is so that you can ground something that you otherwise couldn't (usually through a test equipment ground).

I guess you could argue that making a chassis safe to touch is another use but I would say that is just the same as the above statement except you are a series resistor !

dc
dave cox is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 4:47 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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.