Dangerous troubleshooting

[Shock]

A good video for beginners to watch and pick apart. It shows how you can easily get into trouble quick if you are not confident/competent with test equipment and mains voltage wiring.

Can you figure out what is happening while watching the video? Can you see any mistakes being made or how to troubleshoot differently?

[fubar.gr]

I stopped watching at the part where he says he runs the whole bench from an isolation transformer.

Isolation transformers should be used sparingly, ie powering a specific item at a time. When lots of different equipment is powered at the same time, there's more chance one of them accidentally earthing one side of the secondary, and then it's the same as not using an isolation traffo at all.

[Nomsot]

I examined it to the end.

[chris_leyson]

Is that radio in the first video an AC/DC chassis ? 
And he should earth that flaky VTVM.

[Andy Watson]

A good video for beginners to watch and pick apart. It shows how you can easily get into trouble quick if you are not confidant/competent with test

Confident is NOT the same as competent - as the first 18 mins of the video illustrate. Trusting to "imagined" connections, or lack of, is definitely over-confident. This guy seems to rely on safety devices rather than understanding the principles.

I think we have a potential Darwin Award candidate in the making

[RGB255_0_0]

Very dangerous to be nonchalant about what your devices are connected to.

Running on an isolation transformer doesn't mean, at all, that there is no contact to the systems' ground (the Earth):

If your equipment becomes in electrical contact to anything connected to anything bonded to what is ultimately connected to Earth, it's no longer isolated. Which is what he's done when connecting the power supply to the sig gen.

I cringed so much when he just plugged in those expose wires in and started waving his arms around.

[chris_leyson]

Thanks for posting this Shock  Definately Darwin Award candidate Andy  Just beacause you run your bench from an isolated supply doesn't mean everything is isolated, the live and neutral on every piece of eqipment is connected to the live and neutral on every other piece of equipment. Connect up an old AC/DC radio and the chassis is going to sit at half line voltage or more ! And what is the "light bulb" thing all about. 

[Stephen Durr]

Mr. Carlson's lab has a nice video on isolation transformer ground issues:

[Shock]

what is the "light bulb" thing all about.

He doesn't have a multimeter with a low input impedance (LoZ), so he is using the bulb to load down ghost voltages (capacitive coupling) in order to get an accurate reading. In itself ok but his setup carries it's own risks if you notice the mess on his bench.

[vk6zgo]

       !!!!!

It gave me a headache watching that!

I kept expecting him to get "out of sync" & grab a handful of "bities".

And the lead clamped in the vise with the ends splayed out.......
OK,I have done that,but very gingerly,turning everything off between any changes,even to the point of saying to myself "Power down!---Now!".

He didn't use two IEC connectors because he didn't want to mix them up!
How about a bit of coloured tape on one!

Then he farts around with his test light for ages---AAAARRRRGGGGHHHH!

Finally,at 27minutes in,he does what everybody else would have done at the outset,& checks the "isolation" transformer input to output with an ohmmeter.

Unbelievably,he then messes around with the VTVM & stuff & ignores the transformerless radio "Elephant in the room".

I think the original problem was due to the fact that some transformers are made where the object is not operator safety,but the elimination of ground loops in some equipment,hence earth is connected to one side of the secondary.
There is a Youtube video showing just this,---can't remember whose.

Mr. Carlson's one is very interesting.
As an ancient person,the thought of running the Oscilloscope rather than the DUT from an isolation transformer never really occurred to me as an option.

Many of us Greybeards in Oz were used to DUTs with  built in transformers.
To us,the idea was use isolation transformers to offer the same degree of safety we were accustomed to!

[MarvinTheMartian]

I cringed so much when he just plugged in those expose wires in and started waving his arms around.

Couldn't believe the way he nonchalantly put his fingers near the bare mains wiring!

That's the point I stopped watching - I couldn't take anymore, thinking when he was about to zap himself!  (He did post the video, so he must have come out of it alive.)

PS Not sure what the crackling was when he moved his 'test bulb' and other wiring - sounded electrical but it may have been his mike (if it was a wearable one).

[chris_leyson]

Thanks Shock, at this rate there will be more than ghost voltages.

[G7PSK]

He farts around with every thing but never shows us his non-isolating isolation transformer, my guess is he has a voltage stabilisation auto transformer, the type with an extra winding with a capacitor. An hour and half wasted on what should have been a few minutes.

[tggzzz]

An hour and half wasted on what should have been a few minutes.

That's par for the course with videos. They stroke the creator's ego, are fast to make and slow to view - which is good for the creator and bad for all their viewers.

Fortunately they are soon lost and forgotton, since unlike text, they are unsearchable and therefore ungooglable.

[tautech]

As an ancient person,the thought of running the Oscilloscope rather than the DUT from an isolation transformer never really occurred to me as an option.

Many of us Greybeards in Oz were used to DUTs with  built in transformers.
To us,the idea was use isolation transformers to offer the same degree of safety we were accustomed to!

There's several similar tricks to achieve scope isolation, most shouldn't be shared on a public forum for fear of those that might attempt to mimic the tricks.........well they'll only do it once. 

[Ian.M]

One more serious hazard is if you are working on equipment that uses HV (AC or DC) e.g. a burner ignitor, or CRT EHT supply.  If you accidentally draw an arc from the high voltage supply, the chassis can develop a high potential of the opposite polarity which is likely to be greater than your isolation transformer's voltage rating.  Result: one or more FUBARed isolation transformers, and after recovering from the nasty zap you just gave yourself, if you don't Hi-Pot test the isolation transformers before carrying on, you'll have a good chance of ending up in the morgue.

[vk6zgo]

One more serious hazard is if you are working on equipment that uses HV (AC or DC) e.g. a burner ignitor, or CRT EHT supply.  If you accidentally draw an arc from the high voltage supply, the chassis can develop a high potential of the opposite polarity which is likely to be greater than your isolation transformer's voltage rating.  Result: one or more FUBARed isolation transformers, and after recovering from the nasty zap you just gave yourself, if you don't Hi-Pot test the isolation transformers before carrying on, you'll have a good chance of ending up in the morgue.

Sorry,but I really can't see that happening with  a CRT EHT supply.

Even in large screen colour TV's the EHT just isn't that well regulated---as soon as the quite small capacitance of the CRT is overcome,the EHT supply voltage falls rapidly.

The EHT potential exists with respect to chassis.
All an arc does is complete a circuit which is normally completed by the load--more current,yes,but one side cannot change voltage w.r.t. anywhere else other than points within that circuit.

In any case,if the chassis is separately earthed,it cannot change its potential w.r.t the Mains wiring,& if it isn't,any potential difference is floating w.r.t earth & cannot damage transformer insulation.

[Ian.M]

It depends what you draw the arc to.  If its *YOU* and the set is on a floating supply, unless you are standing on a thick insulating mat, or other good insulator, you've just grounded the final anode and applied tens of KV across the isolating transformer inter-winding insulation.   If there's a crack in the LOPT EHT connector and you are probing round the area or you are careless securing the ground clip of your discharge stick or EHT meter to the aquadag contact spring its quite easy to do . . . .

[Ian.M]

A good isolating transformer is either split bobbin or has an earthed interwinding screen.  Put 10KV on its secondary relative to earth and there is a high chance of a flashover to the interwinding screen or the core, which is likely to compromise the insulation.  The odds of it happening aren't that high, as large screen CRT TVs/Monitors are fairly rare nowadays, so its probably sufficient to just be aware of the need to perform an insulation test on the isolating transformer after any incident involving a HT discharge to anything other than the D.U.T.
Protection for the isolating transformer would involve adding 1.5KV gas discharge surge suppressors between both ends of the secondary and mains earth, but this reduces its overall isolation rating.

The only time I saw something similar happen was when a scope probe with ground clip was being used on the primary of a TV SMPSU.   The scope probe lead trailed against the LOPT EHT connector, which was defective.  It flashed over to the probe lead braid through its insulation, and, as the TV didn't have an aerial connected, the circuit was completed by the SMPSU's optocoupler breaking down and blowing off the board.   It also took out a lot of other semiconductors, writing off the main board.  The scope survived, and the probe was salvaged with some amalgamating tape round the damaged insulation.     

As its sometimes necessary to run up a CRT EHT supply with the final anode lead disconnected (and hopefully safely isolated - we favoured a carefully cleaned straight pint beer glass with the EHT lead clothespegged to the rim so it couldn't jump out , with a thick insulating mat under it) to diagnose a gassy tube, any mistakes can lead to the isolating transformer getting zapped 

[Shock]

[Shock]

[Andy Watson]

Oh dear, another 93 minutes! Have you watched it? Could you tell us the ending, please   Does he actually make any useful conclusions?

[ModemHead]

The low-battery indicator on his DMM has been on the whole time.  Looking forward to a 3-hour multimeter troubleshooting session.

[Ian.M]

AAGH!!! Its too painful to watch.   IMHO Youtube should have a takedown policy for content like that

When I was a full-time repair tech, we had per bench position, one fixed isolating transformer protected by a circuit breaker on the secondary, wired to a fixed switched dual outlet, with its faceplate spray-painted bright yellow and with no connection to the isolated ground pin (apart from the other socket on the same outlet).  We also had one off of the same setup in a portable wooden case, (*NOT* an earthed metal case) for when floating test equipment was unavoidable, or if we had a field service callout. 

There was also an isolated aerial socket carrying UHF TV and VHF FM radio per bench position, also painted yellow.   For isolated Satellite receiver testing we used isolated signal leads on the LNB input and the modulator output, but that was a fairly rare requirement.  We also had a battery powered TV test pattern generator that could be used as an isolated AV signal source.

As such there was one clearly marked isolated supply per technician, strictly for the D.U.T. ONLY.  If the only connections you had made were to yellow sockets or using yellow tagged cables, you knew the D.U.T. was fully floating.   All plugs for bench equipment were clearly labelled to minimise the risk of plugging something into the wrong supply.

Scopes were normally earthed, and everyone was well aware that the ground clip introduced an earth to the D.U.T.  We had a plastic cased battery scope for when a fully floating measurement was unavoidable.

The only possible refinement would have been to have three 4mm socket/binding posts per isolation transformer, connected to true Earth, secondary side Neutral and Secondary side Ground to allow the isolated supply's ground to be bonded to neutral or to true Earth if required rather than left fully floating.  I wouldn't want a switch to select the grounding options as the default for bench service work is fully floating, and any other configuration needs to be bloody obvious + impossible to do by accident.   As was, we used a power cord terminated with a terminal block plugged in the second socket on the isolated outlet if we needed to patch the grounding. 

[Shock]

The low-battery indicator on his DMM has been on the whole time.  Looking forward to a 3-hour multimeter troubleshooting session.

Ah didn't notice, I was kind of ignoring anything the multimeter said anyway as he was using those fine tip probes and never making good contact, which was another problem.

[rsjsouza]

Apart from the low battery, the DMM he is using is not very reliable... I have the Triplett 9005, which is the OEM of the UT-51 and its rotary switch is very flaky.

Thus, both the VTVM, the UT51 and the self-made leads are unreliable... Oh well...

[vince53]

I'm setting up an "electronics lab" for experimenting, etc, since being reenergized by Dave's videos. I'm a retired master electrician and hold an Extra class ham license.

I have a Variac and isolation transformer in a rack mounted chassis that I built 40+ years ago and will get it set up on the bench.  My old analog scope has the ground pin cut off and interestingly I just bought an old B&K function generator with a missing ground pin.  As an electrician I'm well aware of potential shock hazards from improperly grounded equipment.

When I get my isolation transformer set up I will not connect the ground wire to the outlet receptacle but the hot and neutral from the secondary will be fed through a ground fault circuit interrupter (GFCI).  These are used many places in the home for personal protection and are approved for feeding and ungrounded circuit.

From memory, this is how they work.  The hot and the neutral wires run through a current doughnut (no ground connection).  As long as all the current flows through these two wires, the sensor sees zero amps.  When some of the current travels outside of the circuit (you have your hand on a cold water pipe and touched a hot chassis) the imbalance is sensed and the circuit is opened.  The trip current is between about 5 and 10 milliamps, not enough to kill you.

So no matter how badly things are messed up, you won't get killed.  Also, if you do have a leak to ground you will be forced to to find and repair it.  I only intend to use the isolation transformer for the DUT when appropriate and restore the grounds to my test equipment.

Vince

[Andy Watson]

From memory, this is how they work.  The hot and the neutral wires run through a current doughnut (no ground connection).  As long as all the current flows through these two wires, the sensor sees zero amps.

But ...

the hot and neutral from the secondary will be fed through a ground fault circuit interrupter (GFCI).

Since the secondary is isolated there will be no imbalance in the L and N currents - therefore the GFCI will not function.

[mos6502]

Using a UNI-Turd meter for troubleshooting is like using a circular saw for trimming your pubes. The first thing I would do is sell most of that old junk and buy a Fluke 87V.

[vince53]

Andy,

If isolating the secondary for the GFCI to trip a number of things to go wrong.  Such as: DUT has an the chassis hooked to one side of the line and and a non-isolated piece of test equipment has the ground clipped on to the chassis.  There is now wiring in the chassis that is 120vac above ground.  If you touch that wiring and came in contact with an earth grounded object, The GFCI would open and you'd still be alive.

GFCI's are for human protection and they would work on the primary or secondary of the isolation transformer.

The GFCI maintains shock protection while legally allowing you to not connect the equipment ground.

Vince

[Ian.M]

The GFCI will not trip.  The isolating transformer secondary winding is an insulated wire with no way for current to enter or leave it other than via the ends.  Therefore no matter what you do downstream of a GFCI connected to the secondary, including clipping an earthed scope ground clip to isolated Neutral, and a Presto Hot Dogger between isolated Live and the nearest water pipe, you cannot get a current imbalance through it, so it cant trip. (I am neglecting the leakage current via the inter-winding capacitance as that will be an order of magnitude or two below the minimum GFCI trip current.)

The ONLY way to get a GFCI to trip on an isolated supply is to strap isolated Neutral to isolated Ground before running Live and Neutral through the GFCI.   Then any faults to grounded chassis in the D.U.T. will trip the GFCI. However that doesn't improve your personal safety.  All it does is serve as a half-arsed substitute for properly inspecting and electrical safety testing the D.U.T. before first plugging it in, and if you habitually plug in suspected faulty devices you don't know the history of without a safety inspection you are a Darwin award candidate waiting to happen.

OTOH feeding the rest of your bench that isn't on an isolating transformer, from one or more GFCIs with lighting on a separate circuit is worth doing and provides a considerable safety improvement over having a non-GFCI feed to your bench.

[Shock]

Ian can you think of any more additional risky scenarios than listed below?

HV/EHT, HV/EHT arching over, introduced earth, live chassis or heatsinks, neutral tied chassis, live via bench, live via test equipment, live while touching neutral, non discharged caps.

[Ian.M]

There are plenty more workshop hazards, but if you restrict it purely to electric shock hazards, and exclude specific hazards like working on camera flash circuits as subsets of HV or non-discharged caps you've already listed, what about your physical reaction to a non-lethal shock? e.g. jerking away and slicing yourself your on a sharp chassis edge or pulling a heavy rack of test equipment down on top of you.

I aso emphasise the necessity of having sufficient lighting on a separate circuit to the RCD/GFCI.  If you trip the RCD and now have to extricate your hand from the D.U.T in total darkness without encountering anything sharp, charged or hot, there's a significant risk of injury.

[vince53]

Ian,

It seems from your response you won't be easy to convince.  I'll get a GFCI recept over the weekend and see if I can simulate it.

Vince

[FlyingHacker]

It's a shame that companies decided to call these modern devices Isolation Transformers, as they are not fully isolated. The output ground is still tied to the input ground (which in itself is not horrible), and worse, the output neutral is tied to ground.

You either have to buy a real bench isolation transformer, or go in and fix it yourself.

Only the DUT should be isolated. Everything else should be grounded.

[OilsFan]

SDG did some good videos on it too where he specifically mentions the ground.

[tggzzz]

It seems from your response you won't be easy to convince.  I'll get a GFCI recept over the weekend and see if I can simulate it.

Excellent! Anybody that relies on a safety mechanism without having tested it is at best, um, "optimistic". I have  a mains plug fitted with an internal "leakage" resistor, just to check the safety devices do trip as I expect.

Please let us know the results - they will benefit us all.

[vince53]

When I drew out my scenario I could see the GFCI would not trip and all the current would flow through the sensing doughnuts.  I've stripped down my transformer/Variac setup and will be installing the GFCI on the front panel.  I'll do some testing to see if it works on setups that keep the equipment ground intact.  I'm using a US class A GFCI which is rated to trip at 4 to 6 ma.

Vince

[Shock]

When I drew out my scenario I could see the GFCI would not trip and all the current would flow through the sensing doughnuts.  I've stripped down my transformer/Variac setup and will be installing the GFCI on the front panel.  I'll do some testing to see if it works on setups that keep the equipment ground intact.  I'm using a US class A GFCI which is rated to trip at 4 to 6 ma.

Be interesting to see your results.

[Shock]