If you are making a custom setup then also consider adding inrush limiting for the Variac. Variacs are notorious for tripping breakers on start-up due to their high remanent flux caused by their tapewound structure. The output breaker is a good idea.
The RCD would only work for the primary winding, since the secondary is isolated
If you get one built you may as well get a 120V tap on it so you can quickly use it as a stepdown transformer. You also need to consider what ground/earth wiring you have present at the output.
You have many more ways to kill the power from the mains on an RCD-protected circuit if you were to get into contact with a live wire.
Most of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.
at which point the MCB should kick in (though you'll still get a jolt).
A MCB on the output provides no protection against shock as the trip current is far far higher than that likely to cause electrocution, so its only useful to prevent equipment damage and fire.
QuoteMost of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.
See, my problem is that I'm repairing household goods: and getting all up inside them Depending on the product I can sometimes get away with just powering it from a bench supply and bypassing the mains side of things entirely - other times its not convenient to do so, or there are too many rails for me to power manually (rarely), or the problem is with the PSU itself - and of course, sometimes you just need more than the 60V my bench PSU can supply!
Its for cases like that I was thinking the isolated transformer would be invaluable - is that the case? Or would a good RCD a better option?Quoteat which point the MCB should kick in (though you'll still get a jolt).QuoteA MCB on the output provides no protection against shock as the trip current is far far higher than that likely to cause electrocution, so its only useful to prevent equipment damage and fire.
Aye! I realised that a little after I said it - a typical MCB trips at what, 3 Amps? 5? 13? Even with a 1-amp MCB, if you get to the point where you have an ampere flowing through you - cutting the power isn't going to help much (unless you happen to have a fondness for BBQ Engineers).
It doesn't take much to trigger fibrillation either - something meager like 30mA if I recall! (which, come to think of it, is below the tripping current for most RCDs - which, surprise surprise, is why you take precautions to avoid getting a shock in the first place )
No offence taken
You're very right in your concern though! I've been thinking that a bit to myself as I've been replying to things on this thread if I'm being honest.
On the other hand - I generally power things from the bench supply if I can - it's a rare occurrence that I actually have something live exposed on the bench - the problem being that sometimes something dicky comes through that needs to be probed while either while powered up, or even while powering up - which was why I was inquiring about extra protection
--EDIT--
In fact, given the rarity of having to actually deal with live mains - I could probably get away with just refusing to repair anything I can't power safely from the bench PSU. That's probably a safer option overall.
The best piece of practical advice in this thread was to hook up your test probes with the DUT powered doon, then switch it on, take measurements, switch off, move probes etc...
This shit bites.
If you have to work on live mains equipment (and some times you do), do as mentioned above, and throwing a 5mA trip RCD (used on medical gear) on the supply wouldn't be a bad addition.
Stay safe, Brother.
Most of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.
Unfortunately, its common practice to connect grounded testgear to a D.U.T. fed from an isolating transformer.
From experience, a qualified electrician in the UK is the last person you want to ask about electrical safety.
Though I've met a surprisingly large number of electricians who say they've had a shock before.......
Most of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.
RCDs are not suitable as primary protection against electric shock. Sure they help reduce the risk of death, but people have still gone into fibrillation from contact with an RCD protected circuit. A 'working' RCD only guarantees a maximum disconnection time not any kind of current limit.
Isolation transformers however are a very useful tool providing you understand what you are doing. I would never substitute working on some live gear with an RCD instead of an isolation transformer - they just aren't reliable enough.
Another problem I see is distance to the distribution/switch board comes into play with ground protection and that can add delay. Making an additional ground/earth is a no according to some wiring regulations (depends where you live of course), they are also a proximity hazard as well.
The problem is in the understand what you are doing part. An RCD/GFI at least prevents a lethal/prolonged current to flow. An isolation transformer can easely be defeated by grounded test equipment or give a false sense of safety. The only safe way to work on mains (or any high voltage circuits for that matter) is to use isolated probes and never ever touch the circuit.
Now you are not making sense. The delay is in the order of nanoseconds where the speed of impulses through your nerves is in the several tens of meters per second ballpark so it takes a pulse to go from your brain to your toe tens of milliseconds.
No it doesn't. Conditions can be such that you sustain an electric shock with a current that is not sufficient to trip the RCD, yet be greater than the threshold to cause muscles to contract or be enough to cause cardiac arrest. An RCD won't trip at 20mA, but you'll have a hard time letting go and will lead to death.
QuoteNow you are not making sense. The delay is in the order of nanoseconds where the speed of impulses through your nerves is in the several tens of meters per second ballpark so it takes a pulse to go from your brain to your toe tens of milliseconds.
Sorry, I'm not understanding why you brought up nerve impulses?
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.
VERY important distinction!
We really need a sticky post outlining the facts and issues of bench safety in the first post, this comes up all too often and we get people like the "I touch the live mains all the time and I don't die" guys who just distort reality to new players.
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!
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'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).
What is more simple than: don't touch it when powered/charged and use a differential CAT rated probe to measure?
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.
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
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.
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.
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 10Then 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.
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-.
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...
Here is some RCD testing jump to 28:00 and wait for the results to come in.
No visual check to ensure power has been fully removed.