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Power - Earth-RCD's- ISO/TX's and workshops.

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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Refugee » Wed Feb 20, 2013 10:58 pm

We do a lot better with our safety than the consumer products industry in many ways.
Good safety components that are durable that are tested on a regular basis being the finest example.
On consumer goods Mr twiddler will get in and bypass a safety switch that has fallen to bits for no good reason and the safety will go back 20 years and the product will soldier on for a good few more years.
I have seen it done several times and have in the past done it myself in a case where the safety component had almost set the appliance on fire when it failed.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by crackle » Mon Jul 29, 2013 11:34 am

I have recently been giving a lot of thought to safety whilst working in my workshop, particularly as I am often alone in the house.
I have been reading through this thread all morning and still not finished, but I have made a drawing of what I have at the moment, and come to the conclusion that an isolation transformer is providing no protection to me. It protects only secondary test equipment such as scopes which you are connecting to the radio you are fixing.

Warning
If you use an Isolation transformer on your radio under repair it WILL render any RCD device ineffective and you will have NO protection from the RCD either in your workshop or back at the consumer unit.

Conclusion;
Replug my workshop supply with out the isolation transformer but through a local RCD which will be in addition to the RCD on the house consumer unit. (this would not have prevented my recent mishap, which was purely me being stupidly complacent and picking up a chassis with 2 hands whilst still connected and switch on.)

Question
Would it offer any benefit if a 22k resistor was used across live to earth, with switch to isolate it, to add a 10mA leakage. Thus setting a bias on the 30mA RCD and making trip when an additional 20mA leakage was reached.
Or is this barking mad.

Please can somebody correct me if I am wrong.

Thanks
Mike

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by crackle » Mon Jul 29, 2013 12:07 pm

Is possible the house RCD would trip first if it has lots of equipment all connected to it, each with 1mA or so leakage. e.g. already biased.

Mike

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by crackle » Mon Jul 29, 2013 2:09 pm

Just carried out some tests by connecting a 10k resistor between live and earth. The result was both the local workshop RCD and the main house RCD tripped. I tested with 20k resistor and neither tripped.
That seems to show that in my circumstances about a 25mA fault should trip the RCD's.

Mike

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by crackle » Mon Jul 29, 2013 3:07 pm

The lowest resistance I can measure across my wet fingers with a DMM is about 50k ohm. Which equates to about only 5mA at 240v.
Unless the skin resistance reacts differently at 240v, I dont see how this is going to trip a 30mA RCD which trips at about 20 - 25mA.

Have I got the wrong idea.

Mike

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Michael Watterson » Mon Jul 29, 2013 4:33 pm

my 25V ceramic capacitors read >1 G Ohm.

They go pop with 230V though. So do Neons without a limit resistor. So when I was ten I thought the solution was 8 neons in series. A little knowledge is dangerous, I could multiply 8 x 60, the voltage on a lit one with resistor was about 60. Almost all of them popped.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Red to black » Mon Jul 29, 2013 4:46 pm

Just a quick comment on RCD's tripping current.

A 30mA RCD when tested at 1/2 Idn (I delta N, is it's nominal tripping current) or 15mA, it should not trip at all.

When tested @ Idn (30mA) it is tested twice, once on each half cycle, usually designated on the on the RCD testing equipment as 0 degrees and 180 degrees, depending on which standard it (RCD) is certified to, it should trip within either 200ms or 300ms.

This basically means that it could trip anywhere from 16mA up to 30mA, in practice I find they usually trip around circa 20 to 22mA, this is due to manufacturing tolerances.

They are tested on both half cycles because they are usually more sensitive to one or the other half cycle, that is they trip quicker on one half cycle than the other.
Just to add, when testing the slowest trip time is noted on the test certificate (worst case).

It is then tested again twice @ 5X Idn (both 0 and 180 degrees) or 150mA, and it must trip within 40ms (again the slowest time is recorded on the test sheet).

This 5X Idn test is only carried out on 30mA (or below) RCD's used for protecting sockets for general or outdoor use or for general additional protection (mainly, but not solely domestic), ie. it is not carried out on any of the higher than 30mA rated mA RCD's nor on the time delayed types.

An RCD will not protect you against a live to Neutral shock, as there will be no imbalance, what current is leaving the live will be returning on the Neutral (in actual fact there may be enough earth leakage through yourself that it will trip, but this is far from guaranteed, and I would not like to try it to see).

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Red to black » Mon Jul 29, 2013 5:58 pm

Thanks Jeffrey,

My post is a bit garbled too. :ccg

It basically refers to general use RCD's/RCBO's, and that is the general gist of it.
I also meant to say in the post above
for protecting sockets for general and/or outdoor use
but I ran out of editing time by the time I noticed. :bba

There are of course other more specialised types of RCD that are specifically for DC use, as normal AC types don't work properly with DC, these are becoming more prevalent in domestic use due to the solar/PV industry.
Some other specialised types are more suited to pulsed DC, these are typically found in hospitals and the like for use with certain medical equipment.

In fact one type of no-trip Earth Fault Impedance instrument (D-LOK), deliberately introduces a DC injection current to the RCD to stop it tripping whilst taking the measurement.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by crackle » Mon Jul 29, 2013 6:23 pm

Red to black wrote:An RCD will not protect you against a live to Neutral shock, as there will be no imbalance, what current is leaving the live will be returning on the Neutral (in actual fact there may be enough earth leakage through yourself that it will trip, but this is far from guaranteed, and I would not like to try it to see).


I am aware that I would not normally be protected from a live to neutral shock by an RCD. This was the case in my recent accident, and the shock lasted for 3 seconds maybe more. As I had managed to drop the chassis by the time my wife appeared upstairs and I was sitting back down in my chair there was not a lot to indicate what had happened other than the smell of scorched flesh, this was the first thing she commented on.
If there is a chance that some stray charge may leave a body by another route other than down the neutral, then maybe it would be sufficient to trip an RCD, and a 10mA RCD would be the best hope of minimising damage in the event of an accident in a workshop.
I know I should not have any false hope, and that safe working practice is the only answer, but I think I will try and locate one to replace the 30mA RCD I have.
Thanks guys for the information.
Mike

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Red to black » Mon Jul 29, 2013 7:06 pm

Hi Crackle,

As Jeffrey pointed out discrimination is a problem with RCD's (which one trips first), some technical books say they should be 1/3 rd value down stream, ie. 30mA cascaded to a 10mA, but this does not work in practice unless the upstream (nearer the supply) is a time delay type.

We are precluded from using time delay types for general protection (domestic final circuits and socket outlets) by the regs, and besides a TD type for final circuit protection rather negates the purpose , not that I am suggesting you meant this, I am only adding this paragraph for clarity. :aad

From giving you the test procedures a couple of posts back, even a 10mA type would need 50mA to trip in 40ms, and if a heavier leak is present the 30mA may still trip first, and it still only gives Live to Earth protection .

As you found out earlier, even an isolation transformer only protects against a live to earth fault, and not between the lives (as there is no Neutral only two isolated lives wrt earth from an isolation transformer).

These two safety measures can only go so far in mitigating risks, and can in fact give a false sense of security.
It is far better IMO to have safe working practices (one hand in your pocket), and also practice safe isolation (unplug).

Personally I prefer working with an isolation TX and one hand in my pocket, and double pole switched sockets for bench use, btw not all 13A socket outlets are DP switched, in fact most are single pole switched.

RCD's are electro-mechanical and can and do fail (or stick*), plus the fact that the RCD requires a current flow through your body to earth! to achieve disconnection whereas the isolation transformer does not.

* this is why RCD's are supposed to be checked with the integral test button quarterly

Don't get me wrong RCD's have their uses and can be life savers, as do isolation transformers, but neither is a 'magic bullet'.

You can buy all of the protective measures you like, but if you still come into contact with 230V AC across your chest via your arms and legs you are dicing with it.

This is not a dig by any means, and I probably don't need to tell you that you have been lucky, but more for other people reading.

Be safe.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Terrykc » Mon Jul 29, 2013 9:56 pm

Mike, something to bear in mind is that, in the unlikely event of finding yourself in the same situation again - please assure me that you won't! - if you can bring the device you are holding into contact with anything remotely earthy, the RCD will trip.

Obviously, as has been covered several times, this won't help where an isolation transformer is in use but you will probably have been surrounded by earths the other day - soldering iron? signal generator? oscilloscope?

How quickly could you have dabbed the chassis on any one of these - if you had had the presence of mind to do so?

Time is of the essence, as the saying goes, so here is a salutary tale ...

Shortly after I started work I encountered Wally, when he turned up in his invalid carriage - remember those fibre-glass tricycle machines?

In the past, Wally had been an engineer for the company I worked for - until he got a shock ...

I was told that he went to a house one day and took the back of the TV he'd come to repair. This, of course, meant unplugging the aerial, so when he got the back off he went to plug it back in again.

Now this was in the days when lots of sets had unpolarised two pin mains connectors and, in this case, the chassis was live. Wally somehow came into contact with the chassis at the same time that he had the aerial plug in the other hand and, on this occasion, the aerial was earthed.

Not able to break free, he yelled to the customer to pull out the mains lead - but she panicked and ran out of the back door to get her husband, who was at the end of the garden ...

I assume Wally's disability was due to muscular damage or damage to the nervous system due to the delay.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by crackle » Mon Jul 29, 2013 10:36 pm

Terrykc wrote:Mike, something to bear in mind is that, in the unlikely event of finding yourself in the same situation again - please assure me that you won't! - if you can bring the device you are holding into contact with anything remotely earthy, the RCD will trip.


That is worth noting, thanks Terry.
Cheers
Mike

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Alistair D » Thu Apr 03, 2014 1:08 pm

Mark Hennessy wrote:Wow - this has opened a can of worms. I'm not surprised :=D

It's important to remember that just because Employer A did X back in the day, it doesn't make it right, safe, or recommended for current (sorry!) practice.

There are three rules that are utterly rigid:


1. One DUT per transformer secondary.
2. All Class 1 test equipment to be connected to mains earth - earths cannot be lifted.
3. The mains earth should NOT be carried through to the isolated socket.


I'm not sure how much explanation I should attempt to give, especially as it's already been explained (and questioned) on this thread. I'll attempt to explain fully, but as briefly as possible:

1. You are using an isolation to remove the connection between the DUT and the incoming mains so that you can connect earthed test equipment. As you know, mains Neutral is nominally connected to mains Earth at some point, depending on what arrangements you have (another complex issue!). If the mains is full-wave rectified, the live chassis (negative output of the diode bridge) will be at what is colloquially called "half-mains". When you analyse it, you discover that with respect to Neutral, the negative side of the rectifier travels between around +0.6 and -320V, averaging at around -160V. Clearly, this can't be connected to mains earth via a 'scope probe without a fight!

If you insert an isolation transformer, you have broken the connection between the AC input terminals of the bridge and mains Earth. So now, you can ground any part of the DUT you wish. You'll probably want to ground the negative output of the bridge; if so, the secondary winding of the transformer is now floating up and down instead of the negative side of the bridge.

With all that understood, it should be 100% obvious why we have the "1 DUT per transformer" rule, but in case it isn't, ask yourself what would happen if a second DUT had a different PSU arrangement - e.g. half-wave rectification. In this case, you'd be expecting to ground the chassis so that you could connect a 'scope, but the chassis - which would normally be connected to mains Neutral - is now violently moving up and down relative to mains earth because of the first DUT you connected.

Put simply, an isolating transformer provides a floating output. As soon as test gear is connected to a DUT, the output is no longer floating.



2. It was common practice to lift the earth of test equipment, but this practice must be wholly condemned. Quite apart from the performance issues that might occur, the equipment was built to Class 1 standards, and expects to have the metal case firmly grounded so that in the case of an internal fault, the mains fuse can blow before a user receives a shock (Class 2 equipment doesn't have an earthed case, so has higher standards of internal insulation).

So with the earth lifted and the scope probe firmly clipped to the live earth, the whole of the 'scope is at "half mains" for a PSU with an input bridge. Surely I don't have to explain why this is dangerous? Even if the external controls and casework are all plastic, the knobs might have metal grub screws, and the probes will have BNC plugs on the end. Don't. Just don't.



3. Whether to carry the earth through to the DUT socket is often debated. But it shouldn't be - it's pretty clear-cut.

Suppose the DUT has an Earth-Neutral swap in the mains lead. The device appears to be dead because current can't flow through the DUT. But, the case of the DUT is now connected to one end of the secondary winding, and the other end is connected to mains Earth - in other words the case is now live!

It's BBC practice to provide a 4mm binding post connected to mains earth, so an engineer can choose to make the required earth connection before any test gear is connected - this protects the gear against unexpected accidents



All of this is explained in a BBC EGN - point 3 is explained with diagrams in Appendix 1

http://www.bbc.co.uk/safety/pdf/safety- ... v2-1nm.pdf

I hope that this answers the questions about what should be done. Never mind what was done in the past!

Mark


Did anyone download this EGN(4) as it no longer seems to be available from the BBC?

Al

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Cathovisor » Thu Apr 03, 2014 2:35 pm

From the BBC website:

Electrical Safety Guidance Notes (EGNs)

The BBC has previously published a series of electrical safety guidance notes. These are too technical to be of use to the majority of BBC users so, with the exception of the note on Portable Appliance Testing have been withdrawn from this site and are held in on an archive on the BBC Safety Teamsite.

If you have a specific need to consult these then please contact BBC Safety. However, you should note that there is no mechanism to keep these updated or reviewed.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Michael Watterson » Thu Apr 03, 2014 3:10 pm

Daft.

I suppose Libraries shouldn't have text books either as they will be too technical for most BBC users.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Alistair D » Tue Apr 08, 2014 11:03 pm

Still no reply from the BBC!

Here is my latest question.

http://www.tortech.com.au/isolation-tra ... cd-testing

Can anyone shed some light on how these transformers are wired differently to a normal isolation transformer?

Al

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Red to black » Wed Apr 09, 2014 4:50 pm

Alistair D wrote:Still no reply from the BBC!

http://www.tortech.com.au/isolation-tra ... cd-testing

Can anyone shed some light on how these transformers are wired differently to a normal isolation transformer?
Al


That transformer drawing is slightly misleading, the primary side, wired as shown if plugged into an RCD protected circuit would instantly trip it!
I believe the part that is hand drawn on the primary side is meant to represent the supply system itself (TNC-S) rather than be wired that way at the transformer.

That picture shows a fully floating secondary supply, and that part is correct in that an RCD won't work properly as configured, or at least not protect against a L-E shock.
To be honest I don't see the need for an RCD on the secondary side of an isolation transformer in normal use anyway, the protective measure is "Electrical separation".

To be able to test RCDs on the secondary side of an Iso tranny you would need to re-create the N-E link and mimic the primary side TNC-S on the secondary side, and reference one side of the output to Earth.

The only real use I can think of for such a small transformer like that was if you were needing to test a load of RCDs and did not want to trip the primary circuit feeding it! *(also see further down)
The transformer in the link is nothing special, it would just the output that is reconfigured, you could do this yourself anyway.

*Actually Jeffrey mentioned doing precisely this very recently, although I am still not fully sure of the reasoning behind it, unless it was to stop the workshop from tripping the house supply RCD.
It may have some uses when connecting Earthed test equipment to an item you are repairing, but then the shock hazard to earth returns, hence the RCD on the secondary.

We have had huge debates in the past about the merits of such a scheme, and while it does have some merits, it also has some drawbacks, in one case you lose electrical separation and on the other you need an RCD.

I will add my thoughts later. :qq1

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Alistair D » Wed Apr 09, 2014 10:04 pm

My reason for asking about this stems from half way down page 2 of this thread.

viewtopic.php?f=5&t=8080&start=25

The problem is that the RCD does not trip. I simulated an AC/DC set with an isolation transformer and an earthed scope lead connected to the chassis. In reality what I did was to connect earth and neutral on the lead to my Megger CBT3 RCD tester then plug it into the Isolation TX output(The output earth is connected to mains earth) I cannot make the RCD trip as I'd hoped. This thread also explains the problem

http://www.theiet.org/forums/forum/mess ... adid=31769

In researching the problem I came across the reference to using an Isolation transformer with an alternative configuration for testing RCDs and wondered how it was done.

This thread also has some information.

http://www.vintage-radio.net/forum/show ... post673636

Al

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by GlowingAnode » Wed Apr 09, 2014 10:07 pm

Hi Al, have just replied to thread over on channel 2.
I offer similar reasonings to Baz, ie. testing RCD's without tripping upstream devices.
Rob.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Red to black » Wed Apr 09, 2014 11:21 pm

I will draw the circuit for you later Al. :qq1

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Red to black » Wed Apr 09, 2014 11:49 pm

Here is a scrappy drawing knocked up in paint that would be suitable to use an Iso TX for testing RCDs.

The primary windings are shown connected to a normal circuit that may or may not be RCD protected at source, this does not matter too much as if it is RCD protected at the source (primary side) the RCD under test at the secondary side won't affect the one on the primary side.

Normally the output of an isolation transformer neither side is live wrt to Earth, in other words both sides are live and there is no Neutral as such, on my drawing I have added the normal colours as a stripe on the secondary side to show the "virtual" designated L,N and E, these are designated as such to indicate the connections to the RCD under test.
As far as the RCD under test is concerned these secondary connections act in the same way as the normal supply would act. The RCD cannot "see" the N-E link that is upstream.

It is also possible to connect the real E to the virtual E, but this is not actually necessary to prove the RCD under test, this might be added if you were using the transformer for other purposes.
Attachments
Possible RCD test Transformer.png
Last edited by Red to black on Thu Apr 10, 2014 12:09 am, edited 1 time in total.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Refugee » Wed Apr 09, 2014 11:56 pm

Surely items with an isolating transformer in them such as scopes and amplifiers already have a local floating secondary that is often connected via a bridge rectifier to a chassis that is locally connected to earth.
Like with an isolating transformer this will not trip an RCCD if the supplies are touched.
Putting an RCCD on the secondary of the isolating transformer with a local earth at the transformer will work until you connect an earthed bit of test equipment. This will trip the RCCD because the test equipment earth will short out one side of the RCCD.
On an isolating transformer the ends of the secondary will adopt live and neutral status according to witch one is connected to chassis unless the set has a bridge rectifier as this will make them both live at half voltage.
You are just getting a known safe earth on the chassis for test purposes and nothing else when using an isolating transformer.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Red to black » Thu Apr 10, 2014 12:01 am

Ref have a look at how TNC-S systems work :qq1

I posted the drawings way back in this thread, The circuit I have drawn above is purely for testing RCDs so that the test did not trip the Normal supply to the workshop out each time the test is carried out, it is actually isolated from the workshop supply.

I was trying to answer Als specific question on the device in his link. :thumb

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Refugee » Thu Apr 10, 2014 12:22 am

Yes the RCCD test set will work with a 12 volt transformer and a couple of parking lights as the things are currant operated and are of a low enough impedance to pass full supply currant to the load.
Many RCCDs do have a "parking terminal" for the earth wires that is not electrically involved with the device.
Our electrics is indeed fed with 2 core and has an earth pin in the ground connected via an ELCB.

 
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Re: Power - Earth-RCD's- ISO/TX's and workshops.

Post by Red to black » Thu Apr 10, 2014 12:33 am

Ref the above circuit is using a 1:1 isolating transformer, this was drawn only for testing RCDs, it mimics a real TNC-S supply.
The RCD does not actually need an earth to work, it works via any imbalance of current of what is leaving one terminal and retuning on the other, it cannot see faults prior to it's connection, only what is after or "downstream" of it. It won't work on a fully floating supply.

We just normally use it for Earth protection due to the way all public supplies are configured by the supply companies earthing the star point (Neutral) of the supply transformer.
Last edited by Red to black on Thu Apr 10, 2014 1:01 am, edited 1 time in total.

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