Isolating Transformer (UK)

[Mark Hennessy]

I've read that BBC document but it is basically trying to regulate stupidity (also note it refers to 'rules' from 1989 which is more than 25 years ago).

So what has changed in the last 25 years that is relevant to this? We still have Live, Neutral and PE as far as I know.

What is more simple than: don't touch it when powered/charged and use a differential CAT rated probe to measure?

We get that you like differential 'scope probes, but they are relatively expensive, and aren't to be found in every repair and maintenance workshop. And they have problems of their own.

I really don't see why people still try to defend the use of isolation transformers with more do's & don'ts than the 10 commandments which people also don't follow.

1. Only one DUT connected to the output.
2. Don't carry the earth through to the output socket.

Pretty sure that's fewer than 10

As I keep trying to indicate, the only problem with isolation transformers is people's understanding of them. In an electronic workshop, they are about allowing the safe connection of earthed test gear to a DUT - nothing more, nothing less. They are not about changing the probability or severity of an electric shock.

Default live-working practice is to use an RCB (and an MCB for over-current, obviously). The use of an isolation transformer is an occasional requirement for those specific tasks that require it (live-side of a SMPS being the classic example). A differential probe might do a reasonable job as well, but it's an either-or choice - there's nothing wrong with (correctly!) using an isolation transformer.

[nctnico]

I really don't see why people still try to defend the use of isolation transformers with more do's & don'ts than the 10 commandments which people also don't follow.

1. Only one DUT connected to the output.
2. Don't carry the earth through to the output socket.

Pretty sure that's fewer than 10

Then you should apply for a job at the BBC because you can reduce a 9 page document about safety to two sentences. 
I see already 3 requirements for labelling an isolation transformer and then 5 more for using it and 3 more about maintenance.

And the icing on the cake (copied &pasted from the BBC document): In most cases the RCD offers greater protection than a safety isolating transformer.

Ofcourse there are use cases for an isolation transformer but those are for using 230V equipment in moist/wet circumstances (*) and not for repair and R&D scenarios.
* In moist and wet circumstances you have to ask yourself if you aren't better off using battery or compressed air powered tools.

[Cerebus]

I don't use an isolation transformer when working on live equipment. I have a strict policy of doing the following though which is adhered to religiously:

1. Unplug, isolate.

A useful old acronym to remember here: SIDE
Switch off.
Isolate.
Dump (any charge holding circuits such as capacitors)
Earth (those same circuits so that any recharge from dielectric absorption gets dumped and any accidental reconnection of power trips breakers)

2. Attach test probes/equipment.
3. Plug in, power up.
4. Observe measurement without touching the unit.
5. Goto 2.

If your measurement equipment is properly earthed and PAT tested it should be fine.

I don't own differential probes nor do I want to spend any cash on them, so I used an inexpensive AD8130 with two normal 10x scope probes and a secondary switchable attenuator. Works fine, CMRR drops off pretty quick at about 50MHz and noise creeps in so you need to use LP filter in trigger and it'd probably explode if you put more than 500v diff through it but it does the job well enough. I'll detail the design at some point. Cost about £10 in total and runs off a 9v battery. The worst possible current leakage from mains is about 500uA with the design.

[Mark Hennessy]

I really don't see why people still try to defend the use of isolation transformers with more do's & don'ts than the 10 commandments which people also don't follow.

1. Only one DUT connected to the output.
2. Don't carry the earth through to the output socket.

Pretty sure that's fewer than 10

Then you should apply for a job at the BBC because you can reduce a 9 page document about safety to two sentences. 

I see already 3 requirements for labelling an isolation transformer and then 5 more for using it and 3 more about maintenance.

Labelling is an issue for the manufacturer, not the end user. What a peculiar point to make! It is absolutely the norm to label equipment - would you criticise a DMM for having the CAT ratings printed on the label?

I don't see 5 usage requirements. Over and above the 2 points I listed earlier, I suppose you can add the one about visual inspection - which you'd do with anything you use, surely?

Finally, the maintenance points apply to any piece of electrical equipment that you plan to plug in and use (visual inspection and periodic PAT test) - they are exactly the same thing you'd do with any bit of mains-powered gear, whether it's an isolation transformer or not.

And the icing on the cake (copied &pasted from the BBC document): In most cases the RCD offers greater protection than a safety isolating transformer.

Yes, and I have never disputed that - clearly you are not reading my posts properly.

For the 3rd time:

In an electronic workshop, isolation transformers are about allowing the safe connection of earthed test gear to a DUT - nothing more, nothing less. They are not about changing the probability or severity of an electric shock.

So absolutely, don't expect an isolation transformer to increase your safety. This is the big misconception that I frequently try to explain to people, and I find it amusing that you are arguing with me about this, because we both agree about it.

Where we differ is in your assertion that isolation transformers are completely worthless (and historical), and that an expensive differential probe is the only way. Whereas I assert that used correctly, an isolation transformer and a conventional 'scope probe is just as good - perhaps better in some scenarios. But people must know how to use them safely, and I'm happy to pass on that information.

Ofcourse there are use cases for an isolation transformer but those are for using 230V equipment in moist/wet circumstances (*) and not for repair and R&D scenarios.
* In moist and wet circumstances you have to ask yourself if you aren't better off using battery or compressed air powered tools.

Well, I have already given the example of 110V site transformers (with a grounded centre-tapped secondary). 230V versions also exist, as I said in my initial post. Those are intended to reduce the severity of an electric shock, but they are not intended to allow you to connect test gear to a live chassis - let's not confuse the two separate scenarios.

[Delta]

Well, don't take my word for it:
www.tek.com/dl/51W_10640_1.pdf

Oh, and I do take safety very seriously. When I was a teenager a good friend of mine electrocuted himself while tinkering with electricity.

That says not to use an isolation transformer to float the instrument, it absolutely does not say that an isolation transformer should not be used to supply the DUT!

VERY important distinction!

You didn't read far enough! In the table on the next page it says not to use an isolation transformer to make floating measurements -period-.

I did indeed read as far as that table - the whole table refers to instruments NOT DUTs!  (or should that be DsUT?)

The text from the preceding page should make it crystal clear that Tektronix are in no way saying that the use of isolation transformers to power DUTs is "An Unsafe And Dangerous Practice And Should Never Be Done!" :

Floating An Oscilloscope: A
Definition
“Floating” a ground referenced oscil-
loscope is the technique of defeating
the oscilloscope’s protective ground-
ing system – disconnecting “signal
common” from earth, either by defeat-
ing the grounding system or using an
isolation transformer.

Never attempt to defeat the protective grounding system of your oscilloscope by using an isolation transformer...

(emphasis mine)

Seriously mate, I think you've misunderstood Tek's gist there...!

[Shock]

Here is some RCD testing jump to 2:28:00 and wait for the results to come in.

[steverino]

Here is some RCD testing jump to 28:00 and wait for the results to come in.

Actually, the 240v shock is at 2:28:0, line on left hand, ground on right.  This fella might not be around to reach middle age.  Even after the shock, he keeps picking up the live board by the transformer in his left hand while holding a grounded panel in his right.

[Cerebus]

No visual check to ensure power has been fully removed.

Ooh! Product opportunity here for someone with access to a cheap injection moulding process.

Shooters use a device called a breach flag to ensure that weapons are handled safely. It's just a bright orange piece of plastic that has a rectangular 'flag' on the end of it and a more weapon specific part at the other end. It's inserted into the breach of a pistol or rifle in such a fashion that it's impossible to close the weapon's breach and therefore impossible to accidentally load or fire the weapon. Breach flag present - weapon is safe; breach flag absent - treat weapon as loaded and dangerous.

We need a 'breach flag' for IEC sockets. You remove the power cord and insert a moulded plastic plug that has a stalk with a flag on the end, the flag sticking up above the instrument/DUT shows it's safe and the plug part is a reminder to check the instrument for safety before replacing the power cord.

[RGB255_0_0]

https://youtube.com/watch?v=XBsQ3sZ45Fk

[Ian.M]

That video is *NOT* a good one.  See: https://www.eevblog.com/forum/projects/isolation-transformer-and-variac-safety/ which discusses it.

[SteveyG]

That video is terrifying - he seems to be oblivious to the fact he's handling a powered up PCB. 

I did a video a while ago on my isolation transformer - I mentioned floating DUTs or test equipment, but thinking about it now, it's quite a dangerous thing to suggest to the public - Isolation transformers and other protective devices REQUIRE you to understand exactly what you're doing and exactly what is at what potential. Almost an impossible thing to explain in video format.