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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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    Sparkingchip‍ yes, that was my concern. There's no information as to whether this is RCD protected, which would depend on the age of the installation etc. It's possible one or more pieces of equipment may have a serious leak to earth. Removing the CPC would then cause the current to find whatever path it can. If that's through the trace of a circuit board in an expensive piece of equipment...ouch.


    A leakage clamp meter may be indicated for use in the first instance... then the suggestion is doable
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    perspicacious:
    It won’t go away will it, surely if the problem is leakage current flowing along the signal cable screens disconnecting the appliance earth will just increase the current flowing along the signal cable screen?


    Back to my observation earlier in this topic that asked:

    I do wonder why, if the earth leakage from audio equipment is producing its own problem of "pleasurable noise" (sic), a better design of amplifiers is appropriate? Is it a case of the equipment design being the author of its own misfortune and reliant on an earth path to dump its byproducts?


    And doesn't appear to have been challenged that the design poops on itself and all over those with sensitive hearing...............


    Regards


    BOD


    It's not the audio equipment itself in a studio (or on stage) ... it's a mixture of the number of interconnected components, some of which can't be anything other than common-mode connected, but the whole potentially picking up stuff from elsewhere.


    A few years back, BS EN 50310 actually had supply arrangements ranked effectively "not good" (TT), "better" (TN-C-S), "best" (TN-S), although to be honest that's only really relating to the performance of noise filters that include Line to Earth and Neutral to Earth "noise sinking", and it's a whopping generalisation too, which is probably why it doesn't say that any more.


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    I am sneaking a peek at the IET Temporary Power Systems and IET Practitioners Guide books, which seem to reinforce the idea that the issue isn’t the CPCs, but the screened signal cables.


    Using isolating transformers in the signal cables seems to be the real answer.

    09cd425b85ee6f8abe8c24cff73cd3a7-original-20200819_231358.jpg
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    It’s interesting that a transformer is being used as a protective measure there, which I can’t say I've seen much on stage, usually you’re looking at ADS, in a small venue you’d be more inclined to use a transformer to provide dedicated power to the entire audio system if you can, but that’s definitely for noise reduction not shock protection. A plug and play power conditioner is primarily isolating transformers, you get a fair bit of RFI and EMI filtering in them as well these days but alas it’s usually not a perfect solution, you’d be surprised the hums and hisses and crackles that persist or come and go for no apparent reason even with the best of setups. 


    As has been stated, screened cables that interconnect equipment that is otherwise sharing a common Earth via two or more different paths does cause a fair amount low hum, typically it’s solved with a ground lift function of a DI box or stage cab, which can break the alternative path of the screened cable, they also massively help reduce noise problems by converting high impedance signals (say from an electric guitar) to a low impedance signal which allows mixing consoles to receive the signal directly even with very long cable runs. Sometimes ground lift makes some things sound worse, such is life. 



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    The "hum" can be dramatically reduced by providing a single earth connection to the location, and providing multiple bonding connections to local earth bar once there.


    Yes, isolation transformers can help reduce "hum" for guitars, which are common-mode with Earth, in the manner shown in the IET Guidance for temporary power systems, but the type of transformer I was talking about works in a slightly different way ... to help reduce the impact of common-mode "noise" contribution from mains equipment within the studio in the first place, by turning the single-phase mains from common mode with Earth, to Differential Mode, so a lot of the "noise" will cancel, in the manner shown below (and yes ... overcurrent protection is missing on the primary in the lower diagram, but it's only an illustration, not a design ...)

    How a centre-tapped transformer might reduce common mode noise in final circuits


    Provided you bring a dedicated feed into the studio (with optional large CSA reinforcing earth from MET to a studio "earth bar"), and ensure you don't connect earthing in the studio to other parts of the property, except for these two incoming conductors, there should be fewer "hum" problems on the mains. Guitarists would still have the option to use an isolating transformer.


    From experience playing guitar, removing the mains earth completely with an isolation transformer can cause "pops" and "clicks" as fingers move on the fretboard, but I guess that a connection to the "normal mains" earth could be made through a suitably high impedance to help discharge the static. But also you need to be REALLY careful with this, if the amp is Class I, because it might get a "fortuitous earth" if the amp is interconnected with anything else that's earthed, via guitar signal leads (or maybe these days, comms leads)

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