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Old 21st Jan 2023, 11:55 am   #1
Simonpiemon
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Default Isolator Transformer Query

So I purchased an isolating transformer off ebay to prevent electric shock should I touch an incorrectly wired/live chassis as well other projects. I checked the Earch and Neutral were not connected to each other on the secondary which they aren't. I've also checked the resistance between primary and secondary windings and cannot register anything on the Avo. Now I gather the 240v secondary should be 'floating' not live and neutral just floating. Now the issue I have is when I use one of those cruddy electrical neon test screwdrivers on the secondary windings it lights!? Not as bright as on the primary windings but it still lights which has me a bit stumped. Am I missing something obvious? I'm assuming I'd get a shock of I were to replace the screwdriver with my finger. Any help would be much appreciated.
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Old 21st Jan 2023, 12:18 pm   #2
Roger Ramjet
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Default Re: Isolator Transformer Query

Hi Simon,

First things first, I would suggest you use your AVO to check for any AC voltage between each of the secondary winding terminations & a proven earth ?

There could be leakage between the winding's (dependent upon design) but this would need a high voltage insulaiton test aka megger to diagnose.

Rog
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Old 21st Jan 2023, 12:30 pm   #3
GrimJosef
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Default Re: Isolator Transformer Query

Does the transformer have an earthed inter-winding screen ? If it doesn't then perhaps stray capacitance between the secondary and the primary is causing the secondary to flap up and down at some fraction of the AC mains voltage.

Those neon screwdrivers routinely light up if mains is connected to one end but 'nothing' is connected at the other. They rely on the stray capacitance between you and earth to pass enough current to generate a visible glow. As you'll know, since you're holding the screwdriver, while that current lights the neon it isn't enough to give you a tingle, let alone a dangerous shock.

If you tack the secondary to earth via some relatively high-value resistor - say 100k - you may find the neon no longer lights. That would indicate that the secondary's source impedance, via the stray C to the primary, is high enough to limit the available current to a low level. If it does still light then that means that the secondary is capable of delivering at least a few milliamps to ground and you might want to investigate that further.

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Old 21st Jan 2023, 2:31 pm   #4
Lucien Nunes
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Default Re: Isolator Transformer Query

I would actually expect the neon screwdriver to light from one or both terminals of an isolated secondary. It only takes a fraction of a milliamp - less than it takes to feel as a shock - which can easily flow through the stray capacitance from the secondary to everything else: primary or interwinding screen, iron core and surroundings. If the transformer is screened, the neon current could be completely local to the secondary and doesn't point to leakage across the isolation barrier.

A proper test would be to put a 500V DC insulation tester (Megger) across pri-sec, pri-frame and sec-frame. This will distinguish between capacitive leakage that is unavoidable, and substandard insulation which indicates a fault.

As you hinted in your wording, the neon screwdriver is deprecated these days. In the electrical industry it is basically a no-go and I confiscate them from any team members if found. The non-contact indicators such as the Voltstick can be misread too, and are often frowned upon by electricians. However, as they can detect things that cannot be detected any other way, I consider them valuable and teach our people how to use them properly and interpret their indications.
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Old 21st Jan 2023, 4:20 pm   #5
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Default Re: Isolator Transformer Query

Quote:
Originally Posted by Lucien Nunes View Post
As you hinted in your wording, the neon screwdriver is deprecated these days. In the electrical industry it is basically a no-go and I confiscate them from any team members if found. The non-contact indicators such as the Voltstick can be misread too, and are often frowned upon by electricians. However, as they can detect things that cannot be detected any other way, I consider them valuable and teach our people how to use them properly and interpret their indications.
Lucien,

I'm interested in your views on non-contact voltage detectors. I've seen these used where I believe a proper test lamp/indicator and prover should be used - to confirm that things are not live.

Would you agree that non-contact testers can confirm that things are live but not the reverse?

PMM
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Old 21st Jan 2023, 4:24 pm   #6
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Default Re: Isolator Transformer Query

I would want to test an isolation transformer bought for testing and problem solving purposes very carefully, especially if it came from an uncertain source. You occasionally trust your life to these things, sometimes without realising it.

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Old 21st Jan 2023, 5:19 pm   #7
Lucien Nunes
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Default Re: Isolator Transformer Query

Quote:
Would you agree that non-contact testers can confirm that things are live but not the reverse?
Exactly. They are no substitute for an approved 2-pole voltage indicator and such a device (and its proving unit) is mandatory for confirming safe isolation. An AVI is reliable and predictable as it cannot accidentally be set to the wrong range, its relatively low input resistance is not fooled by capacitively-coupled 'ghost' voltages on isolated cables etc. But OTOH it's only any good when you have a circuit. A hypothetical light switch with permanent line, switched line feeding an empty lampholder, and a broken or absent CPC, creates a situation where an electrician might test between each conductor and each other, get no readings and assume dead, when in fact it is still fully live. In this situation, the non-contact indicator is a lifesaver, as the missing earth / neutral reference is replaced by the user's stray capacitance. The same is technically true of the neon screwdriver, although they have reliability and insulation integrity issues that make them a very poor second to the non-contact.

A situation where the Voltstick wins hands down, is testing a cable that is about to be cut mid-span. Unless is is screened (e.g. FP or SWA) the stick will warn of it being live before disturbing it at all. Even if it is screened there is the possibility, if one has any concerns, of dissecting it to access the cores within before finally breaching their insulation. I use my Voltstick for all sorts of tricks. I managed to trouble-shoot a 3-way lighting switching fault without even opening the switches up. I knew which of the three had a bad connection by following the pattern of live and dead metal behind the faceplate. Tracing breaks in Socapex lighting multicores strapped along trusses flown high over the stage - you don't want to be up there longer than necessary - with the ability to follow individual live and neutral cores through the sheath of an 18-core YY.

But I digress.
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Old 22nd Jan 2023, 9:22 am   #8
Simonpiemon
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Default Re: Isolator Transformer Query

Quote:
Originally Posted by Roger Ramjet View Post
Hi Simon,

First things first, I would suggest you use your AVO to check for any AC voltage between each of the secondary winding terminations & a proven earth ?

There could be leakage between the winding's (dependent upon design) but this would need a high voltage insulaiton test aka megger to diagnose.

Rog
Thanks for all the feedback! It's much appreciated. I tested the AC voltage between secondary windings and a grounded terminal and it reads between 20 and 30vac on each terminal.

The chap I purchased it from had done several tests before selling including a Hi Pot test (1500v) between primary & secondary - which was around 0.16mA leakage. Insulation tests (500v) on the primary (input) side, between Live & Neutral to earth and the same on the secondary (output) side - they both read in excess of 510Mohm

Thanks Simon
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Old 22nd Jan 2023, 9:53 am   #9
Roger Ramjet
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Default Re: Isolator Transformer Query

20 to 30AC to earth does not seem right to me from a an earth free secondary winding. Is it strong enough to light a filament bub of same voltage ?

It would be interesting if other forum members are getting similar readings from their own isolation transformers.

Rog
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Old 22nd Jan 2023, 11:03 am   #10
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Default Re: Isolator Transformer Query

If the voltage is from a relatively high impedance capacitively coupled source, what is indicated will depend on the actual resistance of the meter used. This will vary quite a bit depending on the meter and could be anything from Megohms to a few 10s of kilohms.
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Old 22nd Jan 2023, 11:26 am   #11
Simonpiemon
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Default Re: Isolator Transformer Query

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Originally Posted by Roger Ramjet View Post
20 to 30AC to earth does not seem right to me from a an earth free secondary winding. Is it strong enough to light a filament bub of same voltage ?

It would be interesting if other forum members are getting similar readings from their own isolation transformers.

Rog
Thanks, it doesn't seem to light a 12v 2w bulb. What's slightly odd I've noticed it doesn't seem to make any difference to the needle on the avo when I switch it from 1k through 600, 300 etc it is consistently reading around 20vac. When I measure the voltage across both live pins however it measures normal?
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Old 22nd Jan 2023, 12:49 pm   #12
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Default Re: Isolator Transformer Query

Quote:
Originally Posted by Simonpiemon View Post
What's slightly odd I've noticed it doesn't seem to make any difference to the needle on the avo when I switch it from 1k through 600, 300 etc it is consistently reading around 20vac. When I measure the voltage across both live pins however it measures normal?
That's a very good sign.

Remember, an analogue meter measures current. The meter coil, which moves from 0 to full-scale depending on the current through it, has a low resistance. When measuring voltage, a much higher resistance, called the multiplier, is connected in series with it; and essentially, all the voltage is being dropped across the multiplier, so Ohm's Law on the multiplier resistance converts the current reading to a voltage, and increasing the resistance increases the voltage required to move the needle to full-scale.

So if changing the series resistance does not affect the meter needle, then what you are measuring is effectively a constant-current source; and whatever leakage path exists is only capable of delivering a limited amount of energy.
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Old 22nd Jan 2023, 12:54 pm   #13
Lucien Nunes
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Default Re: Isolator Transformer Query

Quote:
I tested the AC voltage between secondary windings and a grounded terminal and it reads between 20 and 30vac on each terminal.
The voltage across what is supposed to be a broken circuit is quite immaterial, just as the voltage to earth of an overhead line is immaterial to a bird sitting on it. What is important is the current, because a high voltage (part or all of the secondary EMF) in series with a high impedance (the insulation resistance and capacitive reactance) behaves as a constant-current source.

Actually, although some of your meter ranges are scaled in volts and some in amps, the meter always behaves as an ammeter just with different shunts and multipliers, which change the input resistance. Therefore what you are seeing is quite normal and indicates a very small current source comprising the secondary voltage and (probably) stray capacitance. Of course, a different meter with a different input resistance will give a different reading, so the actual '20-30 volts' on the dial doesn't tell you much about the transformer. You will get a more useful indication measuring on a current range, because that will be the actual value of current and not just an arbitrary number.

TL,DR; Measuring leakage on a voltage range is misleading and both low and high voltage readings can be expected. Measure leakage current instead.

*Post crossed with@julie_m*
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Old 22nd Jan 2023, 5:44 pm   #14
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Default Re: Isolator Transformer Query

Leon,

Thanks for your helpful and interesting comments. Please continue to digress, it adds interest by giving an insight into your considerable experience.

Simon,

To quantify what Julie m and Lucien have written, on voltage ranges, the pointer on your meter always shows a proportion of movement full scale current, where the movement fsd current is the reciprocal of the "sensitivity" in ohms per volt. From the ranges quoted, its seems you are using a Model 8 Mk V, 6 or 7 Avometer.

The Model 8 Mk 7, for example, has a sensitivity of 2000 ohms per volt on the 30 V (AC) and above ranges, so the full scale current is 0.5 mA and 2%, 3.33% 6.66% of that when indicating 20V on the 1000, 600 and 300 V ranges respectively or 10, 16.7 and 33.3 microamps.

Interesting things these analogue multimeters. When you know what they're saying, they don't lie.

PMM

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Old 23rd Jan 2023, 11:04 am   #15
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Default Re: Isolator Transformer Query

I have to say that I really do not like modern double insulated devices that often put you in capacitive contact with the mains. For me an isolation transformer should include in its remit isolation from capacitive coupling as much as possible - so having an earthed screen between the windings is essential.

When you use any meter or other test device it is absoluitely essential to know what exactly it is doing or the results can be do misleading. My fall-back is always to a multimeter that I built (many decades ago) - because I know what it does!

I recently ran into the most surprising example of capacitive leakage: a recent reincarnation of a box of relays that orgnaises our central heating had the added new feature of LED indicators to show when mains relays were activated. There was a circuit for a planned future change - so imagine my surprise when its relay LED lit up - it wasn't connected to anything, expect that the unterminated wire ran through a long cable with other live conductors. I would never have guessed that modern LEDs were so sensitive that they can light up with almost as little current as a neon might need. The LED with resistor for working with the mains is in parallel with the mains relay. You might think that would quench any misleading behaviour but actually not!

BTW I have a modern non-contact thing that is a part of a clamp meter. It's misleading thing is to respond to changing electrostatic fields so it often frightens me by flashing when I go to pick it up. If used to find a cable by wafting it about it often flashes anyway so generating a lot of concerns for hidden cables that do not exist!
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Old 23rd Jan 2023, 11:35 am   #16
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Default Re: Isolator Transformer Query

I had a new boiler fitted recently, and while the electrics were being relocated and hooked up by the plumber's electrician, I asked him to quickly remove a length of mains T&E that had been 'hanging' out of the kitchen ceiling on top of a cupboard for the past 15-20 years. I'd checked it was disconnected from mains years ago, with the notorious neon screwdriver (), and blanked the end off anyway to ensure no exposed wires, but never actually removed it because that would have entailed floorboards up etc. Now, with the floorboards up & piping all exposed, seemed like the perfect time...

Said electrician promptly pulled his 'non-contact wand' and declared it live We 'discussed' it for 5 minutes, before he finally traced it upstairs and accepted that it was indeed not live, and was actually completely disconnected at both ends. It was simply an old length of cable that had never been removed, routed in close proximity to some of the other house wiring.

As GMB says - When you use any meter or other test device it is absolutely essential to know what exactly it's doing or the results can be misleading.
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Old 23rd Jan 2023, 2:24 pm   #17
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Default Re: Isolator Transformer Query

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If used to find a cable by wafting it about it often flashes anyway so generating a lot of concerns for hidden cables that do not exist!
You need something that draws a tangible current to check, as neon testers and Volt-Sticks will, at best, err towards safety by indicating live, rather than at or about zero potential. This can inculcate a false sense of security as once known about, may be assumed dead, even though it indicates live, and is therefore ambiguous. Testing the tester (Volt-Stick) by rubbing it briskly on one's shirt-sleeve to watch it illuminate does not instill confidence.

I have a 2 Watt LED pygmy bulb in the landing lamp on a two-way switch. If I switch it off at one end it extinguishes. If I switch it off the other end, it glows, but at reduced brilliancy. This is because of capacitive pick-up in the longer three-core-plus-earth wiring from that switch. Putting an AVO 8 across the lamp-holder indicates it to be dead.
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Old 23rd Jan 2023, 4:40 pm   #18
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Default Re: Isolator Transformer Query

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I have to say that I really do not like modern double insulated devices that often put you in capacitive contact with the mains. For me an isolation transformer should include in its remit isolation from capacitive coupling as much as possible - so having an earthed screen between the windings is essential.
Agree, but it's easier said than done!

Suppose you wind the primary, and then put on a single turn of foil which you earth. And then start winding the 240V secondary. The first layer of secondary turns will be fairly close to the earthed screen - separated only by a couple of layers on insulation from the screen. Whereas the last layer of turns will be remote from the screen.

So, the capacitance from screen (earth) to the start of the secondary will be fairly significant, maybe 100pF. Whereas the capacitance from the finish will be much lower - probably around 10pF (to the laminations, which will be a couple of mm away, unless you have wound your bobbin really full).

When powered up, if you measure the voltage between the unconnected secondary and earth, you'll see around 22V on the start leadout and 218V on the finish leadout. And short-circuit current at the finish leadout will be around 7uA, still enough to give a tingle!
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Old 23rd Jan 2023, 5:17 pm   #19
Roger Ramjet
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Default Re: Isolator Transformer Query

This is a theoretical question only.

If the primary winding was powered by a floating earth free supply, then would that stop any capacitance induced voltage [referenced to earth] in the secondary winding ?

Rog

Last edited by Roger Ramjet; 23rd Jan 2023 at 5:19 pm. Reason: add word
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Old 23rd Jan 2023, 5:21 pm   #20
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Default Re: Isolator Transformer Query

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Suppose you wind the primary, and then put on a single turn of foil which you earth. And then start winding the 240V secondary....
That's why you don't do it like that. Safety transformers have separate bobbins so we just need to add enough screening so that the secondary cannot "see" the mains and you can get a good result. Actually it may be enough to earth the laminations if the bobbins are far enough apart.
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