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Old 17th September 2007, 11:15 AM   #1
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Question 240Vac mains isolation transformer

Hi,
all the power socket circuits in my house and garage/workshop have individual RCCBs mostly 30A/30mA.
The only circuit that does not have an RCCB is the main listening room which has a 30A MCB and 4sqmm ring.

What are the implications of adding a mains isolation transformer, at the distribution board, for the listening room circuit?
Would that defeat safety earth?
Should safety earth be left connected to the mains combined Neutral/Earth lug?
Or should it be kept separate with a BIG label warning an electrician of the arrangement?

Since the circuit is limited by the MCB to 30A, would the transformer need to be 7.2kVA or bigger or smaller? If smaller should an over temperature switch be fitted to it? Any other protection that needs to be considered.

If this becomes reality, is the best place for HF filtering: at the feed into the isolation transformer or at the output or at the room sockets or in the individual equipment boxes?

Or forget the thoughts?
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Old 17th September 2007, 12:09 PM   #2
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Hi AndrewT

RCDs operate by measuring the current balance between two conductors using a differential current transformer, and opening the device's contacts if there is a balance fault.
(Wikipedia quote)

Placing a isolation transformer will provide galvanic isolation
But in theory will not defeat the RCD.
I use to repair SMPS's for a living and use isolation transformer with a RCD to protect my CRO and myself from been zapped.
It worked well and yes the RCD did trip from time to time.

I assume you are wanting to use balanced mains. if so you will need to fitt DPDT mains switches to isolate both conductors when
turned off.
Assuming the RFI and EMI is mains born then the filters would be best used in the primary, at the very source so to speak.
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Old 17th September 2007, 02:26 PM   #3
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by The Saint
............Placing a isolation transformer will provide galvanic isolation
But in theory will not defeat the RCD.
I use to repair SMPS's for a living and use isolation transformer with a RCD to protect my CRO and myself from been zapped.
It worked well and yes the RCD did trip from time to time.

I assume you are wanting to use balanced mains. if so you will need to fitt DPDT mains switches to isolate both conductors when
turned off............
Hi,
we used to call them RCDs but for some reason changed the name to RCCB or RCBO. We are talking about the same device.

If the RCD is before the isolating transformer and the output of that transformer leaks to safety earth then the feeds to the isolating transformer are still in balance and the RCD does not recognise the leakage and does not trip.
If the RCD were in the output of the isolating transformer then it will check for balance in the flow and return. BUT, the main reason for not fitting an RCD/RCCB to the listening room circuit was to minimise the number of trips to any equipment that generates a high output offset during shutdown. The Conrad Johnson pre-amp does exactly that and is currently retired until I find a solution to the irregular mains failures that afflict this rural area.

I was not thinking about balanced supply, just the isolation advantage and possibly the reduced HF garbage that will be attenuated by a large EI transformer.

I think ALL our BS1363 switched sockets must have DP switching as part of our approvals process. I'm away to check.

Your experience re safety in the electronics workshop could easily be implimented with a local isolator and RCD after the transformer. That is more likely to be my first modification. Thanks for the thoughts.
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Old 19th September 2007, 01:32 AM   #4
OzMikeH is offline OzMikeH  Australia
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The main point of the isolation transformer is you can contact the circuit at ONE point and not be electrocuted.
RCD would not do much after an isolation transformer.

Effectively the isolation transformer isolates the neutral from the earth. so if you become a path from active or neutral to earth there is not a circuit, therefore no current flows, therefore you keep breathing.

Double pole switches after an isolation transformer doesn't make a lot of sense to me. If you connect any of your equipment chassis to earth (antenna cable) AND one piece of your double insulated equipment has a neutral-chassis fault you are at risk of being electrocuted from the active.

I suggest if you with to use the isolation transformer as a filter you connect the neutrals together and ensure the output socket has the earth wire connected. then CLEARLY label the transformer, you might forget and try to use it as an isolation transformer and kill yourself.

All of the above only applies to MEN electrical systems.

Look at your equipment load, use an appropriately sized transformer for the load, fuse it's input appropriately. Don't bother trying to make it take 30 amps, I doubt your wall socket is rated for 30 amps.

I don't think you will have much success using an isolation transformer to filter out high frequency noise. You're better off making a pair of 100Hz low pass Pi filters, one for active and one for neutral. Put a polyswitch across them for about 300V and you're set. Mine kept tripping the RCD/ELCB/whatever because of the current it was pulling filtering out spikes on the active.
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Old 19th September 2007, 04:10 AM   #5
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I would not use balanced mains, electronics where never designed to support it.

I tried it a few months ago to see if i wanted to implement it permanently. For some reason the RCA commons on my preamp had 60 volts... good thing i was using a tube preamp and power amp. Anyway i fixed that up and the sound was no better other than about a few millivolt less noise.

My computers neutral was grounded, i checked before i plugged it in. Not that anything would have been damaged but a fuse but still.
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Old 19th September 2007, 07:34 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi Oz,
I found a google link for MEN electrical system
http://www.ee-oz.com.au/resources/misc/3633.pdf
But what is it telling me about MEN?
Please explain.

I will re-read your comments a few times to fully understand the implications.
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Old 20th September 2007, 12:09 PM   #7
OzMikeH is offline OzMikeH  Australia
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Essentially earth is tied to neutral at one point in each house.
contacting neutral is not advisable but should be perfectly safe.

Not sure why they do it this way or how common it is around the world. might just be to save money by using single pole switches.

Draw out some rough circuits to help you visualise what I was waffling on about. It should make more sense, then again it might not, I was rather tired.
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Old 23rd September 2007, 03:49 PM   #8
mrwhy is offline mrwhy  United Kingdom
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[QUOTE]Originally posted by OzMikeH
[B]The main point of the isolation transformer


WOW! I havwe been looking for an answer to THAT question for 60 years!

WHY do they connect one side of the mains (the neutral) to earth!. How many millions have they electrocuted that way!

A few mA will not kill you so 100K to earth would be fine!
Or better still a resistor with two back-to-back zeners.

If the unbalanced consumers result in 20 Vac on the neutral, who cares!


In the case of lightning strikes (unlikely in UK as the 230v lines are underground) arrestors can be fitted. The INDUCTANCE is what allows lethal lightning surges, so GROUNDING the wire elsewhere is no use at all!

Best idea would be to have every house fitted with a mains-isolation transformer.

The REASON we use 230v in UK is we have no copper - so it, like all else here, is overpriced.
Cheap copper in US allows 110 v and it is actually quite difficult to kill yourself with 110 volts.
John
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Old 24th September 2007, 07:10 PM   #9
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Quote:
WHY do they connect one side of the mains (the neutral) to earth!. How many millions have they electrocuted that way!

A few mA will not kill you so 100K to earth would be fine!
Or better still a resistor with two back-to-back zeners.

If the unbalanced consumers result in 20 Vac on the neutral, who cares!

In the case of lightning strikes (unlikely in UK as the 230v lines are underground) arrestors can be fitted. The INDUCTANCE is what allows lethal lightning surges, so GROUNDING the wire elsewhere is no use at all!

Best idea would be to have every house fitted with a mains-isolation transformer.

The REASON we use 230v in UK is we have no copper - so it, like all else here, is overpriced.
Cheap copper in US allows 110 v and it is actually quite difficult to kill yourself with 110 volts.
Actually I think you misunderstand some of the 'why'. It starts at the distribution stage. Each single-phase for domestic supply is derived from a 3-phase distribution sysstem - for efficiency AND safety the 'domestic' side of the local transformer has the centre-tap nailed to Earth for safety (what could happen if this 11KV+ transformer flashed-over..?). So the incoming supply is already unbalanced. In the UK and much of Europe, where whole districts are served by a chunky transformer, using PME to attach everything else (in the supplied home) to the same earth potential ensures safety given an unbalanced system.

As for safety w.r.t 240v, take a look at the other regs that govern what you can sell. The only people at risk of a 240v shock to earth are those who meddle with DIY equipment and/or take the covers off things...

Without a hard bond between 'neutral' and 'earth', the two could float a dangerous distance apart, for reasonsas mundane as..the neighbours are drwing a different amount of current to you. Depending on how wet your hands and feet are, that 'dangerous' voltage might only be 20VAC... the mains has very low impedance. That, too, is a legacy of the 230VAC, ring-main norm (whihis a very efficient system - 32A avalable at every socket, given certain assumptions abpout diversity of loading)

BTW US systems using 220vac, balanced, has the centre-tap bonded to earth. Have a good think why.
Which brings me back to the OP:

Andrew - returning to topic - all the isolation transformers boxes I've seen for pro uses (I have a 1.5KVA example in front of me, by Clude Lyons who know a thing or two) preserve the bond to main Supply earth all the way through, presumably in case you get a failure somwhere and could conceivabley, personally, end-up bridging between an undefined local 'earth'' voltage and the Earth potential of the main supply system (like a radiator, in an equipotential-bonded PME system). Insert an earth-line choke, rated to carry the prospective fault curent, if you are worried about noise - but I would NOT cut the hard-earth bond.


PS the inductance issue is a red herring; it provides protection against remote lightning strikes, sometimes , solely because it limits the rate of change of current transients. It does not provide a hard limit to shorts - but then strikes are pretty unlimited in energy
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Old 25th September 2007, 07:11 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
If I'm reading this right then Martin & Oz are saying maintain the distribution board earth wire all the way past the isolation transformer and into the sockets of the isolated circuit.
All Class1 equipment are then bonded back via their safety earth.
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