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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 Red to black » Thu Apr 10, 2014 12:56 am

Al here is a picture of an RCD, as I said what leaves one outgoing terminal must return on the other within the trip rating of the device, otherwise it trips.

Now if you add this drawing to the one I posted earlier, on the output side, you should be able to see that if either of the outgoing connections touch the designated (or virtual Earth) Earth that the RCD under test will trip.

A N-E fault shorts one side of one of the coils (coloured blue), leaving an imbalance through the coils coloured brown, likewise on a L-E fault the fault current would only flow through the Brown coils totally bypassing the blue coloured ones and the load, also causing it to trip.

The test button merely connects a resistor across the incoming L to the outgoing N, creating an artificial imbalance in the coils.
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RCD 01.jpg

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

Post by Refugee » Thu Apr 10, 2014 1:33 am

That is pretty close to what I would expect to find in one.
I would expect to see the test button wire down stream of the switch contact though.

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

Post by Red to black » Thu Apr 10, 2014 1:46 am

These ^^^ are modern (since 1981) current operated devices.
Prior to 1981 VOELCBs were used (Voltage operated Earth Leakage Circuit Breakers), and as their name suggests were Voltage operated, this relied on the installations Earth rising above 45V for them to trip.

These fell out of favour since Bonding effectively rendered them useless by keeping the Earth connection below the voltage rise needed to trip them, or by bypassing them as they had an Earth connection in and out, they are now obsolete.

Back to RCDs, these work by having the two current sense coils (blue and brown on the earlier drawing)wound anti-phase, so with a balanced supply and load the net magnetism is effectively cancelled out, with no net magnetism the trip coil sees nothing and the device works.

A fault to E (or designated Earth, or prior N-E connection in this context) after the current sense coils, either L-E or N-E leads one sense coil to pass more current than the other (as explained earlier), this leads to a net magnetic field in the Toroid, this then sets the trip coil to operate the trip mechanism, effectively tripping the RCD.

As said earlier, the sense coils do not detect anything before the RCD.

Ps. the RCD needs to be pulling some current through it (L and N) to work.

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

Post by Red to black » Fri Apr 11, 2014 12:31 am

Further to this discussion, I would like to point out the drawing I did last night for the RCD test Transformer was the absolute minimum needed to test RCDs, it was drawn to prove the operation of the way RCDs work.

It was actually an isolated floating TNC system, in practice you would connect the incoming Earth to the secondary side as well, this was mainly omitted for clarity as it would not actually influence the test of the RCD on the secondary side, nor would it influence the primary side circuit, If I was constructing this test transformer I would include this Earth connection as shown in this modified drawing.

I should also like to point out that on our mains (public supply) system it is forbidden in this country to make a TNC system, or even a "homemade" TNC-S system by using the supply N to make a protected Earthed Neutral conductor (PEN conductor), only the DNO is permitted to do this on the supply side.
On no account should this be done using raw mains.

I would not see a problem making this circuit up as an item of test equipment, or as a standalone unit to be used in conjunction with a piece of test equipment, I would probably be a bit wary of making a full workshop supply with it though, it sort of falls through the cracks of nearly all of the wiring regs, I would really need to look a lot deeper (regulation wise) before committing myself. :qq1

Ps, I noted that link was an Australian document, in place of "live" on the primary side they used "A" standing for Active, which is their term for live I believe.
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RCD test Transformer with E connectected to secondary.png

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

Post by Alistair D » Fri Apr 11, 2014 9:37 am

There is no bout about it that vigilance is the way forward but we all know that accidents do happen. That is why I decided that An RCD on the output of an isolation transformer would be a little bit of extra insurance.

Currently I am tryng to read up about the properties of the TNS/TNCS earthing systems to find out why they allow an RCD to trip but my locally made earth neutral join wont. I understand why the local join does not allow the RCD to trip but not why the household earthing arrangement does.

Al

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

Post by Michael Watterson » Fri Apr 11, 2014 12:47 pm

But ...
With any PSU using a bridge direct to mains you just want a floating transformer. You can't even assume you can earth a centre tap. You're going to be earthing the -ve rail of the PSU, and unless you are an expert and don't make mistakes interpreting schematics, you can't assume what the relationship between AC in wires and -Ve rail.

Similarly on a live chassis, you can't assume the "neutral" is to chassis.

So if I need an isolating transformer, I only earth the test gear and assume everything is live. An RCD is no help at all really. A 200mA to 2A fuse or fast current trip on feed to transformer (depending on application) is more useful.

An RCD is only really use for ordinary fault conditions not for a workbench, that's my conclusion.

With equipment that IS isolated often there might be no transformer connection that's earthed. An earth if fitted may be somewhere after the PSU and need not be + or - wire of a bridge (sometimes there are -ve current sense resistors in a PSU and earth connects after that point.

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

Post by Red to black » Fri Apr 11, 2014 1:38 pm

My own preferred option is to have a fully isolated and floating supply for feeding the equipment under test, and if necessary I would plug my scope or other piece of test equipment into the same isolated supply.

When using multiple items from a single isolated supply any CPCs (protective earths for the test equipment) should be connected together but not earthed, the easiest way of achieving this is to have say a 4-way socket block connected to the secondary side of the isolation transformer (no earth) and plug both the item and the test equipment into the same socket block.

Now the above is not ideal, as it is possible if using two items of class 1 test equipment to create a short via the CPCs supplying the test equipment and any earthed leads, or cause other problems say by monitoring both the hot and cold sides of a SMPSU, I would mitigate the risks here by using only one piece of test equipment at a time, and being careful where you connect the test equipment to.
I found when using my Hameg scope on this type of set-up to just use one earthed clip off of the scope to attach to the equipment under test, even if requiring both channels.
This set up is a reasonable compromise and is about as safe as you can really expect using a simple set-up.

As Jeffrey pointed out above, by using a DIY TNC-S type of ISO TX and RCD combination you lose the safety measure of electrical separation and end up with RCD protection in lieu, to me by using that method you are back to square one really, and may as well just use an RCD protected outlet directly off of the mains.
About the only thing this may achieve is a lower prospective fault current via the ISO TX.
You are merely replacing one protective measure with a different one (you cannot have both).
The exception being of course is if you were using this type of supply for testing RCDs. :qq1

Jeffrey while you are correct about the DIY type supplies,
by ppppenguin » Fri Apr 11, 2014 7:11 am

We can use an isolating TX to provide homemade versions of various supplies.....Completely floating is IT. Earthing one side of the secondary is TNC-S or TNC which amount to the same thing here

I found that this was common, even industry standard, in say control panel transformers and the like, but this was limited to being part of the equipment itself rather than actually being distributed for use elsewhere. The main reason the secondary's were earthed in this case was for fault protection to the cables rather than for shock protection.
I would really need to think more about that one :qq1

by Alistair D Currently I am tryng to read up about the properties of the TNS/TNCS earthing systems to find out why they allow an RCD to trip but my locally made earth neutral join wont. I understand why the local join does not allow the RCD to trip but not why the household earthing arrangement does.

Al, I mentioned this earlier but I will repeat it again in case you missed it, the RCD requires some current flowing through it L and N for the trip to work, your house RCD probably has some loading already present, Try say running a lamp from the out going side of the RCD while trying to trip it.
Also on this set-up the CNE link is very close physically to the RCD.

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

Post by Cathovisor » Fri Apr 11, 2014 2:50 pm

My setup has always been simple: bench outlets fed from RCD. Any 'live' chassis set fed from an isolating transformer with NO connection to the earth pin on the output socket, but a binding post for earth provided adjacent. Only ONE device per isolating transformer. As per the BBC EGN, and the red safety folder before it!

Bench and floor covered in ribbed rubber matting, and in a dry area.

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

Post by Red to black » Fri Apr 11, 2014 3:08 pm

Cathovisor wrote:My setup has always been simple: bench fed from RCD. Any 'live' chassis set fed from an isolating transformer. Only ONE device per isolating transformer. As per the BBC EGN, and the red safety folder before it!

Bench and floor covered in ribbed rubber matting, and in a dry area.


Yes, yes, we know Cathy, :bba
we have had huge debates in the past on this and other forums, in an ideal world we would all have multiple Iso Txs.

We are discussing the best compromises short of disconnecting the Earth off of the test equipment :qq1

:cca

Most of this is the theory behind various schemas, protective measures, and mains supply earthing anyway, not necessarily the way it should be done.

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

Post by Red to black » Fri Apr 11, 2014 3:12 pm

That's a fair size Jeffrey (ooer! missus), are you planning on running G8's of off of that transformer ? :qq1

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

Post by Red to black » Fri Apr 11, 2014 3:30 pm

We had one about that size at the first TV repair shop I worked at, the one I have now is only a baby portable one, not sure of it's rating as the plate is missing, probably about 250 VA.
ttt:

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

Post by Refugee » Fri Apr 11, 2014 4:32 pm

My bench has the earlier 1000VA lump that RS used to sell that came in a steel box with lugs to bolt it down even though it is so heavy that it will not move anyway.
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DSCF5831.JPG

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

Post by Refugee » Fri Apr 11, 2014 4:49 pm

Mine is staying on the floor with the nice big metered Variac up at bench level.

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

Post by Terrykc » Fri Apr 11, 2014 4:53 pm

ppppenguin wrote:I'm thinking of hanging my 40lb lump of ISO TX from the underside of my bench. I'd better make sure it's well secure :ccf

What's the matter with the floor? (Standing on not hanging from!!!)

The further you can keep the electro-magnetic field of the transformer from sensitive equipment, the better. I'm particularly thinking of shadow-mask tubes here ...

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

Post by Refugee » Fri Apr 11, 2014 5:54 pm

Mine is at the back of a cable storage area with a bench power supply and Variac 3 feet above it.
There is a Trinitron screen 6 feet above.

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

Post by Alistair D » Fri Apr 11, 2014 10:38 pm

Having read the many discussions in this thread and elsewhere I came to the conclusion the the safest method of using an isolation transformer is one per bench and only one item connected to it(only my opinion) From this I realised that when an earthed instrument is connected to the chassis of a live chassis set any shock hazard protection is lost. That led me to wonder if an RCD could be added to the transformer output then some protection could be regained.

Experiments showed that the RCD would not trip. I could understand that but could not work out in a real installation. Jeffrey made a comment that made it all clear. The earth neutral link needs to be upstream of the RCD and not downstream as would be the case with a scope connected to chassis.

Another great idea consigned to the bin. I think I will go back to playing with Leggo. :aai :cca

Al

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

Post by Michael Watterson » Sat Apr 12, 2014 1:11 am

Yes. you got it nailed now. The isolation transformer is just turning a something with no mains transformer into a device with one so you can pick an arbitrary point in that device under test and connect that point to "earth". Hence they can't be shared.

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

Post by Mark Hennessy » Wed May 07, 2014 10:58 pm

Alistair D wrote:
Mark Hennessy wrote: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


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

Al


https://dl.dropboxusercontent.com/u/990 ... former.pdf

All the best,

Mark

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

Post by Alistair D » Wed May 07, 2014 11:19 pm

Mark, thanks for the link.

Al

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

Post by Rebel Rafter » Wed Dec 17, 2014 9:14 pm

Hi, everyone, RR here again. I'm not qualified to give proper advice about mains earthing, that seems to be a subject on it's own. But one thing I would recommend if you haven't done it already is if you're working in a remote shed, as a friend of mine does, then you should definitely have some kind of emergency back up lighting, like the ones in screwfix for instance, just in case you're using say a hot soldering iron in the darkness hours and the power trips out, you could get badly burnt if you suddenly can't see what you're doing. My mate fixes motorbikes in his shed so I fixed him up with an emergency light as he handles petrol out there, as well as having engines running, etc., with hot exhausts amongst other things. RR.

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