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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 15th Sep 2018, 11:36 pm   #21
Phil G4SPZ
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Default Re: Lamp limiter plus Variac tip

I suppose if Al measured the resistance of his own body and built a substitute using resistors, it would be safe to test out his theory!

Another thought is whether the resistance of the body varies with the applied voltage, like our well-known waxy capacitors. Only one way to find out, I suppose...
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Old 16th Sep 2018, 12:44 am   #22
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Default Re: Lamp limiter plus Variac tip

The whole point of my earlier posting seems to have been sadly entirely missed
the two protective measures are entirely different, RCDs are great for Earth referenced supplies ie, public mains supplies, generally different protective measures should not really be mixed, unless there are good reasons for doing so, which granted there are times where this is warranted (as always there are exceptions) sometimes this is unavoidable such as in the case of an ELV (extra low voltage usually up to 55 volts) transformer or psu also that happens to be an isolation transformer or isolated psu at the same time , yellow 110V site transformers that are also centre tapped Earth also fit into this category,

RCDs are not the panacea that they are made out to be, to be sure they have a valuable contribution and can indeed be life savers when used for their intended application ie. Earth referenced mains supplies.
more is not always better ie. trying to mix RCDs with isolation transformers, as the isolation transformer is its own protective measure in its own right and does a good enough job for all intents and purposes.

Fully floating supplies are often used in conjunction with Earth free environments which is yet another protective measure, however these are usually strictly monitored/supervised and are out of bounds as far as a domestic workshop is concerned.

The point still stands that if you simultaneously happen to come into contact with two current carrying conductors (supply and return, or some variation of) unless it is of Extra Low Voltage (ELV yet another protective measure) then you will almost certainly get a belt, and no protective measure can save you from this.

Ps, there are other protective measures available, one is double or reinforced insulation, another two not available/applicable in a domestic environment is placing out of reach or with barriers, again usually supervised
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Old 16th Sep 2018, 10:46 am   #23
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Arrow Re: Lamp limiter plus Variac tip

Quote:
Originally Posted by Phil G4SPZ View Post
I suppose if Al measured the resistance of his own body and built a substitute using resistors, it would be safe to test out his theory!
That was the approach I had in mind. (Incidentally, I wouldn't use the word 'theory' here: it's a test and measurement exercise; a theory may follow as a consequence). But most importantly, there is not the slightest possibility that I would use myself as the 'test resistor'! However, I do appreciate the comments made earlier which expressed a concern for my welfare: thank you.

Al.
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Old 16th Sep 2018, 11:00 am   #24
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Question Re: Lamp limiter plus Variac tip

Quote:
Originally Posted by Red to black View Post
The whole point of my earlier posting seems to have been sadly entirely missed.
Not by me.

Quote:
Originally Posted by Red to black View Post
Fully-floating supplies are often used in conjunction with earth free environments, however these are usually strictly monitored/supervised and are out of bounds as far as a domestic workshop is concerned.
On whose authority? If I choose to configure my domestic workshop - which is totally under my control in that manner - why would that be "out of bounds"?

Aside: Yes, I do realise the risks of such an arrangement. But that is not relevant to my questions as asked above.

Al.
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Old 16th Sep 2018, 11:41 am   #25
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Default Re: Lamp limiter plus Variac tip

Hi Al,
To quickly answer your question/s

As far as the electrical regs go a domestic dwelling is habited by non-skilled "ordinary persons" as such the certain protective measures as listed are forbidden from this environment, even if you are a "skilled person" yourself you cannot use that as a "get out clause" because other members of the household will certainly be classed as "ordinary persons", this part of the electrical regs is normative, therefore it "shall" or "shall not" be done, depending on what particular aspect we are talking about.

if your workshop was a registered business (it could be a room in a house) then you could apply the commercial rules for that room only, where this was the case at one place I worked, there has to be certain other conditions applied to that room however.

I hope this clarifies the situation

Ps. under commercial rules the HSE also forbids live working on your own, there is supposed to be at least someone else present to isolate in case of an accident, administer first aid etc.....etc.

Ps.2 what you decide to do in your own home is not really anyone else's business, just you are then fully liable should something go badly wrong. Please don't take this the wrong way, it is just how it is.
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Old 16th Sep 2018, 1:38 pm   #26
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Default Re: Lamp limiter plus Variac tip

I would have thought that running a live chassis radio directly from the mains, with the chassis connected to neutral, then the RCD will only protect you (ie disconnect the supply and probably plunge the house into darkness after you get a shock) when you connect yourself between the set’s HT and mains ground. If you connect yourself between HT and chassis, a much more likely scenario, then the current flow is between mains live and neutral, and the RCD will not trip.

I’d sooner connect the set through an isolation transformer, then I can be sure the chassis is not live, and I can use earthed test gear without any drama. And I must be careful not to touch the HT, just as I would with a set with a built in mains transformer.

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Old 16th Sep 2018, 2:13 pm   #27
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Default Re: Lamp limiter plus Variac tip

Exactly Stuart,
That is what I have tried to get across, Rcds do have there valid uses. but also lead to a false sense of security, mind you the same applies to an isolation transformer, the point being if happen to come into contact with both feeds of the supply side of things then you get a belt.

We have had many heated discussions in the past which one is safer, they both have pros and cons, the basic problem appears from time to time when people try to mix up different protection schemes in the mistaken belief that it is safer, which it isn't

Edit: in actual fact you are better off with the device under test (DUT) and the tests equipment both fully floating ie, the use of one or more isolating transformers (or battery powered test equipment) , this of course still won't prevent a shock if you happen to come into contact across a supply, which of course makes sense as that is the supply's purpose to power the load.
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Old 16th Sep 2018, 2:29 pm   #28
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Default Re: Lamp limiter plus Variac tip

Red to Black: thanks for your excellent reply; very informative.

Al.
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Old 16th Sep 2018, 3:21 pm   #29
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Arrow Re: Lamp limiter plus Variac tip

Reference post #18.

Test set-up.
Mains isol. xfmr. with 120 - 0 - 120 sec'y. Each 120-v. phase was fed through a 240-v. RCD rated to trip at 30 mA. Each phase and the 0v. connection were brought out to terminals for the test. A test resistor was connected between each phase in turn and the 0v. connection.

Result.
Maximum resistance that caused trip to operate was 5600 Ohms.
The trip operated instantaneously when the connection was made.
At 120-v. RMS, that corresponds to a current of 21 mA.
Same result on each 120-v. phase.

Conclusion.
This RCD is designed to perform its safety function with 230-v. a.c., tripping at 30 mA. Test shows that with 115-v.a.c. applied instead, the trip current is less than 30 mA. Consequently, I conclude that this RCD is acceptable to use in my suggested scenario.

Al.
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Old 16th Sep 2018, 3:45 pm   #30
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Default Re: Lamp limiter plus Variac tip

What would the center tap normally be connected to ?

Lawrence.
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Old 16th Sep 2018, 3:52 pm   #31
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Default Re: Lamp limiter plus Variac tip

Hi again Al
just a heads up, a typical 30mA rcd should not trip at 1/2 IdN (I delta N ie,its nominal trip rating) @15mA, it should trip in less than 300 ms at its rated IdN 30mA, on both half cycles, they are polarity sensitive wrt to trip times, under the 17th ed regs they should trip in less that 40 ms at X5 IdN 150mA at both half cycles, although the 18th Ed has removed the requirement to test at 5 XIdN.
I found by ramp testing they usually trip at about the 22mA mark when functioning correctly.

Edit: they can theoretically trip any where above 1/2IdN and X1 IdN, although in reality they tend to trip at about the 22 to 23mA mark This is of course at the usual nominal 230V rating
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Old 16th Sep 2018, 7:12 pm   #32
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Default Re: Lamp limiter plus Variac tip

All RCDS have some variation with trip currents its a normal and accepted thing . Generally speaking electronic ones trip faster than electro mechanical ones as I have found here I have a plug in 30 mA trip feeding the plug board on my bench under fault conditions it trips faster than the 30 mA trip in the house fusebox. Here i use an isolation TX to power whatever im repairing all test gear plugs into raw mains which is the correct way the way TVworkshops do it. The other way of course is to take the earth off your scope(I didn't say that)
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Old 16th Sep 2018, 9:27 pm   #33
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Arrow Re: Lamp limiter plus Variac tip

Quote:
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What would the centre tap normally be connected to ?
Lawrence.
Typical load to have a metal case or metal parts that can be readily touched by the operator / user. So the 0v. centre tap, via the 'E' pin of the 3-pin plug / socket, is to be connected to that metalwork.

There may be a case for connecting that centre-tap 0v. point to the supply earth, but that is a contentious aspect which has been discussed at length here before but with considerable disagreement and without a definitive conclusion. So let's not go there again on that sub-topic; thank you, everyone.

Al.
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Old 16th Sep 2018, 9:30 pm   #34
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Question Re: Lamp limiter plus Variac tip

Red to Black: thank you for that last informative post. However, one Q. please: do you agree with my conclusion as stated in my post #29?

Al.
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Old 16th Sep 2018, 9:47 pm   #35
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Default Re: Lamp limiter plus Variac tip

Al, just thinking aloud now... my own isolating transformer doesn't have a centre-tapped secondary, but by the same thinking, I should be able to plug in a conventional RCD safety plug into the transformer's output to add the trip facility, shouldn't I? The RCD is current-operated and it senses the difference between the currents through its two active conductors, irrespective of whether the leakage current flows to earth or to anywhere else. It's the imbalance that is detected.
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Old 16th Sep 2018, 10:00 pm   #36
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Question Re: Lamp limiter plus Variac tip

Phil: what path can there be for the 'missing current' to flow back to the transformer and thus by-pass the trip? For that reason, it's not going to work as a trip, surely?

Al.
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Old 16th Sep 2018, 10:21 pm   #37
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Default Re: Lamp limiter plus Variac tip

Yes, of course! No path exists, so there's no need or purpose for the extra RCD in my case.

In my defence, I did say I was "thinking aloud"...!
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Old 17th Sep 2018, 12:06 am   #38
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Default Re: Lamp limiter plus Variac tip

I get lots of ideas, Phil: most of them, but not all, upon a subsequent 're-think', turn out to be non-starters. The really important thing is to detect the imposters before serious resources are deployed - and that may require 'input' from others, for which this very forum is an invaluable resource.
In addition to that - and not necessarily related to radio & electronics - I have two mottos:
1. "Your first idea is never the best one. Let it tick over in your mind for a day or so and a better one will emerge". Similar process to solving crosswords, etc.
2. Before 'diving in', get pencil, paper, eraser and a ruler: make a drawing; do the arithmetic. Particularly relevant to woodwork tasks. "Make your mistakes on paper: paper is cheap; other materials aren't."

Al.
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Old 18th Sep 2018, 5:03 pm   #39
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Exclamation Re: Lamp limiter plus Variac tip

Posts 18 and 21 refer.

WARNING. DO NOT TRY TO DO THIS - IT COULD KILL YOU!

Just out of interest, I made a suitable arrangement* to determine the amount of a.c. current from a 120-v.a.c. r.m.s. source, through one hand of my body and out of the other, at which point my reaction would be "Ouch!" and thus rapidly let go. To monitor the current I used my trusty AVO 8.

Results.
At 1 mA, it was a bit more than a slight tingle.
At 2 mA, it was "Ouch!" - and I rapidly let go.
Currents greater than 2 mA were not used!

I measured the resistance between my two hands: 100 kΩ; 120-v. and 100 kΩ = 1.2 mA. I was surprised that a current as small as 2 mA caused the reaction that it did. And it makes me think a bit about RCDs that have a trip current in the range 20 to 30 mA . . . . .

* The details of which I am not going to describe, in order to dissuade anyone from doing this experiment.

Al.
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Old 18th Sep 2018, 8:59 pm   #40
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Default Re: Lamp limiter plus Variac tip

Thanks for doing the experiment, Al, that's above and beyond the call of duty right enough!

When I was a lad, I used to be able to feel a noticeable tingle from the half-wave-rectified '12 volts' from my model railway transformer. It seems that we become physically aware of currents well below levels at which they become potentially harmful.

The 300ms tripping time allows up to 20 cycles to pass, barely enough time for you to utter the first "o" of "ouch!" before the RCD cuts off the supply. You'll survive, but it may hurt for a while.
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