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Old 4th August 2008, 10:56 AM   #1
johnm is offline johnm  United Kingdom
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Default UK laws - dealing with ground loops?

Hi folks.

Does anyone know if the circuit discribed here is legal to implement in the UK, for a valve poweramp (Dynaco ST70):

http://sound.westhost.com/earthing.htm

I have an earth loop but I don't wish to disconnect any of the safety earths, and this looks like an ideal solution, short of converting all my gear to balanced signal interconnects!

Many thanks,

- John
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Old 4th August 2008, 11:52 AM   #2
Mooly is online now Mooly  United Kingdom
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Hello again John,
You mean the bridge rectifier I take it, for lifting ground impedances. Is it legal ? I am 99.99999% sure the answer is no, for the reason that the earth connection has to be "direct" and meet absolute requirements for minimum impedance etc in the event of a fault. It would fail on all counts.
The earth connection has to be able to carry the full fault current ie live to earth short until the fuse blows, and to be able to do it repeatedly time after time.
That said in practice it probably would be O.K. If a fault occurred diode bridges usually fail short circuit under overload but IT CANT BE GUARANTEED. Do you see the problem. If it failed open circuit the protection would be removed.
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Old 4th August 2008, 11:54 AM   #3
Gordy is offline Gordy  United Kingdom
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As soon as the earth cable enters the unit take it to a good solid ground point on the case (...assuming a metal case of course). If you have a multi-part case make sure each part of the case conducts to that point (...just resistance test it).

The rest is up to you. 'Lifting' the audio ground to break a loop is common practice. The ESP circuit looks comprehensive. Substituting two 1N4007 diodes reverse strapped across each other is usually as good as the bridge though.

If your mains transformer has a steel case remember to ground it to the amplifier case.
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Old 4th August 2008, 12:07 PM   #4
Gordy is offline Gordy  United Kingdom
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Quote:
Originally posted by Mooly
Hello again John,
You mean the bridge rectifier I take it, for lifting ground impedances. Is it legal ? I am 99.99999% sure the answer is no, for the reason that the earth connection has to be "direct" and meet absolute requirements for minimum impedance etc in the event of a fault. It would fail on all counts.
The earth connection has to be able to carry the full fault current ie live to earth short until the fuse blows, and to be able to do it repeatedly time after time.
That said in practice it probably would be O.K. If a fault occurred diode bridges usually fail short circuit under overload but IT CANT BE GUARANTEED. Do you see the problem. If it failed open circuit the protection would be removed.
Refering to Figure 3. on the ESP site (http://sound.westhost.com/earthing.htm) then the ground is made to the case immediately upon entry, which is fine. If there is a subsequent fault the worst case scenario is a power short to chassis ground, which would connect directly to the grounded earth point and the fuse / ELCB would trip in the house distribution board.

If there is a more minor fault on the audio circuit board then the resistor will conduct and the ELCB in your home's distribution panel should trip. If the current is so great as to create a 1.4 Volt drop across the resistor then the diodes of the bridge will conduct.

Admittedly it's been lots of years since I looked at the regs, but I think this set-up should be fine.
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Old 4th August 2008, 12:11 PM   #5
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Gordy's right - but just top make this really clear, the problem really rests with the ambiguous way the word 'ground' is used.

If your equipment has conductive cases they must be bonded to the mains Earth to minimise shock*

Tha 0v side of the circuitry within that case does not have to also bonded to mains earth, but in any system it should be, once, for safety. You can control how and where this is done. If you have only one such Earth-0v bond in a system, you have no 'earth loops' by definition, but you must ensure that any interconnects can carry the propsective fault currents and/or fuse the secondary side of circuit supplies to ensure disconnection in the case of a fault to Earth.

The purpose of ESP circuit is in this latter link, to separate the circuit 0v side from the Mains earth, but in a manner that can carry fault current.


* unless 'Double Insulation' standards are adhered to. Without stringent testing this is not a suitable DIY technique
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Old 4th August 2008, 12:11 PM   #6
Mooly is online now Mooly  United Kingdom
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IN4007 WILL explode under fault conditions. They are 1 amp devices. These were often used as bridge rectifiers in TV's VCR's etc . Seen it countless times. Two bits of wire where once lived a diode.
This is the problem of dubious advice. A huge bridge rectifier may survive or fail at least short so giving continued protection but as said NO GUARANTEES
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Old 4th August 2008, 12:51 PM   #7
Gordy is offline Gordy  United Kingdom
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Diodes exploding before circuit breaker trips? Possible I suppose, and no guarantees as you say. In which case 3 spent on a chunky 35 Amp bridge rectifier might be money very well spent.
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Old 4th August 2008, 01:47 PM   #8
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Just to add another voice that 1N series diodes would be completely unaccaptable. A creative portable appliance tester might find a way to test the equipment in such a way that the 20A test current is passed through the loop breaker, and if 1A diodes were used for this purpose then I hope it's now blindingly clear what the consequences would be.
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Old 4th August 2008, 03:30 PM   #9
Gordy is offline Gordy  United Kingdom
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As an aside: Portable Appliance Testing is completely irrelevant for your personally owned domestic items, even more so if you have built it yourself !
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Old 4th August 2008, 04:05 PM   #10
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So true, but surely it's good practice to build your kit to pass safety standards?
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