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Clean Earth sockets

IronFreely
I’m just wondering about how clean earth sockets (Some of you may call these high integrity sockets) fit into the requirements of BS7671 in terms of their use in domestic homes.


One of my clients has asked me to look into the use of some clean earth sockets to be used to power audio equipment in their home studio, the basic idea is that a completely separate Earth helps eliminate what’s known as an earth hum on audio equipment caused by functional earth leakage of other equipment connected to the same circuit and disturbances originating from the suppliers earthing arrangement. The basic concept seems to be to insulate the cpc from the earth terminal of the socket outlet which then has its own independent Earth electrode, high integrity sockets seem designed to accept and insulate a cpc and a separate earth. 


My concern is that this sort of thing is usually used in a restricted access environment where only IT or audio engineers are going to be around and might be considered trained and competent to understand the risk of introducing an earth potential to the equipotential zone that is not in itself connected to the equipotential bonding. It wouldn’t be a problem if I could guarantee restricted access or that the socket would only supply class 2 equipment, but as this is a home studio it seems a bad idea to have an earthed metal casing of some piece of audio equipment that may be completely separate from all other earthing and bonding in the property? 


id appreciate your thoughts and advice please, I’m confident a separate clean earth will resolve the earth hum problem which I’ve seen work well in theatres (essentially restricted access) before but never in a domestic property.


Edit: I should add that the property is a TT property but because the street is a hodgepodge of TNS and TNCS (I have other neighbouring clients really close by) you should assume the gas and water pipe are throwing in a bit of PME related disturbance.
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    IronFreely:

    Let’s just assume both myself and my client have had a good old listen and carried out a few rudimentary tests to determine if the hum is due to local conditions or just faulty old gear. 


    MHR, thank you for your support.


    My point is that you may be wasting your time, and his money, chasing a dragon which doesn't exist. If there is hum in the output, it must be possible to measure it.


    Are you sure that he does not have tinnitus, possibly due to excessive exposure to sound?


  • 0
    I'm sure it is measurable, audio amplifiers take signals at sub-millivolt levels and amplify them to many amps in the voice coil of the speakers. As such anything that introduces a voltage gradient at the millivolt level between the audio source and its the amplifier input. That could be hundreds of microteslas of stray magnetic field thouh wiring loops of a few square metres, or stray tens of milliamps through hundreds of milli-ohms of cable resistance or both effects in combination.

    In the days when a HiFi installation was all separates, this was common - the fact that the modern box is all in one small box reduces the problem for normal uses. There are ways of routing supplies to minimise both inductive effects and voltage gradients, but it is almost impossible to eliminate totally.

    There are also various nasty techniques like inserting impedances or nonlinear devices antiparallel diodes in the  CPC to reduce low level  circulating currents, but while OK inside a sealed box it is very easy to create a really dangerous system if this is done badly in a distributed system.
  • 0
    Meanwhile the beer fridge compressor just kicked in and is chugging along quite happily.
  • Former Community Member
    0 Former Community Member
    Meanwhile the beer fridge compressor just kicked in and is chugging along quite happily.


    You or the fridge chugging the beer Andy?


    Regards


    BAD
  • 0
    I call it as I see it. I see a person obsessed with dropping ANY amount of hum, even if it's imperceptible, I also see a person being pragmatic.  Pragmatism always wins.  If you are a TT island in a sea of PME, there IS no solution that a customer who doesn't understand the basics can cope with.


    There are solutions, but a guy who doesn't understand the need for electrical social distancing (new term)... isn't going to like it

  • 0
    If the pockets are deep, and assuming single-phase installation, I thought of an isolating transformer to provide a local TN-S system for the studio.


    However, better for reducing "hum" would be an isolating transformer with a 115-0-115 centre-tapped secondary. Earth the centre-tap, and this Earth can be bonded to the TT earthing system. If < 10 kVA you might get away with 20 Ohm to 200 Ohm for the earth electrode resistance. If greater than 10 kVA, additional electrodes recommended to get the earthing resistance down to 20 Ohm.


    Even though the secondary is TN-S, because it's centre-tapped earth, protective devices and means of isolation for on all circuits will require disconnection of all live conductors.


    Why is this a good idea?
    1. Provides effective isolation from protective conductor currents elsewhere in the installation

    • Protective conductor currents from all mains noise filters in the studio will be effectively zero.

    • A "balanced" supply is better for noise reduction than a "single-ended" one.

  • 0
    This is a chuffing good idea! Now that I think about it I’ve seen similar set ups in venues before, a plug and play power conditioner is essentially a 240/240 isolating transformer.

    Ive got two questions if I may? Firstly, presumably suitable double pole isolation can come in the form of a bog standard BSEN 61008 RCD, or are you suggesting that DP socket outlets should be used additionally.

    Secondly can you recommend such a transformer of this nature? I’ve never installed one and the only ones I know of are the sort used in medical locations....


    Thanks, this is the kind of advice that makes this forum a real gold mine, you just have to sift through the occasional troll suggesting the only issue is tinnitus!
  • 0
    Whilst DP switched socket-outlets would be preferable, as I don't see them as being necessary, because the isolation in a socket-outlet is removing the plug, which is double-pole.

    The 2-pole isolation requirement is there for TT as well,, and we can use SP switched socket-outlets for that can't we?


    HOWEVER, a guitarist who opens up their own amp, say to change valves, might assume (erroneously) that the switch provides isolation, so even leaving it TT I might be inclined to consider DP switched socket-outlets?


    Apologies, no handy recommendation for the transformer, but I guess if you posted what your power requirement is, someone on the Forum will have a recommendation for you from jobs they have done in the past.
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    IronFreely:

    Thanks, this is the kind of advice that makes this forum a real gold mine, you just have to sift through the occasional troll suggesting the only issue is tinnitus! 


    That's not very polite!


    I return to the question about the amplitude of this noise. Has it been measured; and if so, is it at a level which is capable of being perceived? If either answer is "no" then any attempt to abolish the noise may be a waste of time and money.


    ETA: we are talking about a "domestic home" are we not?


  • 0
    Chris Pearson:
    IronFreely:

    Thanks, this is the kind of advice that makes this forum a real gold mine, you just have to sift through the occasional troll suggesting the only issue is tinnitus! 


    That's not very polite!


    I return to the question about the amplitude of this noise. Has it been measured; and if so, is it at a level which is capable of being perceived? If either answer is "no" then any attempt to abolish the noise may be a waste of time and money.




    You’re right, that’s not very nice because derailing someone’s query by suggesting its more likely the OP has tinnitus because they haven’t measured a noise to prove they can really hear it is about as usefull as suggesting you should measure ambient light levels to prove the picture you can see on your bedroom wall isn’t a hallucination. Chris I’m sure they have an earth hum, I can hear it, the client can hear it, the clients band mates can hear it, we can all hear it when the equipment is on, we can all tell it goes away when the equipment is off, we can all hear it on recordings made with this equipment, we can hear it has been significantly reduced when plugged into the power supply in my shed which is a TT island, a group of people with something like 300 years of collective experience in music and sound engineering do not need to measure a sound to know they can hear it nor do they need to measure it’s absence to know it’s been eliminated by driving across town and plugging it in elsewhere. The question is NOT IF there is an earth hum, there is, the question is how to reduce or eliminate it within the scope of BS7671 in the location they want to use it. If you don’t like the picture you take it away, you don’t need to prove you can really see it first. In any case amplitude on equipment of this nature is a variable not a constant, measuring it is subjective to controllable gains with the exception of the powered monitors which present with all faders nulled. Chris please if you can’t help with solving the problem please go away and stop suggesting we’re imagining it, it comes across as patronising and rude. Thank you. 


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