Inveterate meddling #104: dedicated earth connections

OK we've outlined the DIY mains spur. When possible, this includes a dedicated earth path for equipment which needs this connection, but this is not without risk. Here we outline the issues to do with this new earth path and we're abandoning our usual 'happy-go-gibbonlike' demeanor to play the role of safety-nazi on this, using lots of red ink and word like 'risk', because the risks of fire and electrocution are real enough for the ignorant.

First off, note we are relating only to the UK mains supply which apart from being 240VAC @50Hz is different in implementation from many overseas installations, not least because we use extra fuses in the the plug of the equipment to be protected (owing to the near-unique Ring Main design) - and chunky polarised plugs including a Mains Earth bond pin - plugs that can only be inserted one way.

By way of a basic introduction: Protective Earthing is primarily and ONLY a safety feature. It does not directly impact the 'sound' of your system. It doesn't primarily affect ' RF' beneficially or otherwise*; it does not 'drain interference' or many of the other dubious claims you may have seen made. This page is purely to illustrate some of the complexity and risks associated with domestic mains supply - Earth bonding arrangements. [* this is solely a function of the design of the elements in your system, including the cables.]

In the event that a piece of electrical equipment becomes faulty such that the mains 'live' conductor leaks to the equipment casing or some other touchable element, the fact that these parts are Earthed (grounded, US). The resulting short circuit means a high-enough fault current flows to ensure that the fuse/primary circuit protection blows, disconnecting the Mains electrical supply. Functionally, this happens by the Earth being a secondary route bonded to the neutral connector. Essentially then the mains supply is unbalanced, with one conductor - 'Live' - 240VAC above the 'Earth' potential and the other conductor - Neutral -being only a functional current return within appliance line cords. Hence the need for plugtops that only fit one way.

In the UK the incoming electrical supply provides the domestic earth connection. This is in the legal ownership of the Service Provider. It is essentially the point where the supply Earth and Neutral are linked together though the way in which this is achieved varies. Traditionally, the raw supply coming in has a separate earth spike into the ground at the point near where the electrical service enters the property. Any fault current makes its way via the earth (literally) back to the centre-tap of the local distribution transformer. This is the 'TT' designation and is still the norm for much older properties and those in remote (non-suburban) areas, as well as much of the US. However, from approx. 40years ago the UK domestic norm turned to using PME, Protective Multiple Earthing, or TN-C-S using the IEE terminology. Here the Earth and Neutral wires are connected together directly at various intervals in addition to the local substation transformer neutral being earthed. At the service entry (the meter box) usually only two wires enter the house - phase live and neutral, and the house earth circuit connects to the neutral here. In this way, earthfault bonding for equipment in the supplied property is assured even if there is an earth line fault elsewhere in the system; a major safety issue.

Whilst a definite safety feature when used as designed, like all things electrical PME presents some risk potential for those tempted to idly bung in a 'hifi' earth. If the house supply is PME then idly adding a local earth spike in addition to the bonded neutral has the effect that your house could end up as the safety earth route for 10-100 houses in the area. For example in the case of a broken neutral elsewhere in area supplied by the same substation transformer the multiple bonding, and all the leakage currents from your neighbourhood's hundreds of bits of inlet-filtered equipment /SMPS now sees your hifi's earth. Do you still want to rely on your earth spike ?

The Right Way

There is nothing wrong with connecting a local earth providing that it and the earth cable connected to it through your house can carry such a fault current. If you choose to add an earth spike it can be done on a PME system but only in this way: it must be bonded to the (PME) earth bonding point at the consumer unit only, using a conductor of at least 16mm2 Officially, and esp. since the introduction of IEE 17th Edition in 2008, even this is deprecated, and the reason why - risk of confusion - is expanded below. Hence the recommendation on the previous page that your new earth and its bonding must be tested by a 'competent person' as defined in the Regulations. For what it is worth note that Russ Andrews' guide to upgrading your mains supply does, or at least to, show a PME system also connected to a local Earth post- and this guide expressly notes it has been reviewed by employees of two separate UK supply companies. That piece of mind is worth exactly what you just paid for it...

NOTE Again - as of July 2008 the regulations governing mains power installtions in the UK have been updated to IEE 17th Edition, which must be consulted before you really plan such work.

The Wrong Way

One apparent alternative in order to provide a local earth would be to connect the new earth to the outlets supplying the system, and disconnecting the service Earth. This is idea is worth exploring further only to highlight the risks it brings. The fundamental problem with DIY earth rods is simple: it's difficult to get even close to the service earth impedance (usually c. 0.2 Ohms)- and the local stake's impedance will vary with soil type, season, rainfall etc. One poster on the Naim Forum connected a digital multimeter between his house earth and a new earth stake in the garden:

AC was more of a surprise (the numbers weren't this high
when I first installed). PD is about 1.5V, and current about 150mA.
I presume this must be 50Hz, but there's no way for me to measure it.
This example earth spike equates to ~10ohm impedance: rather good, actually, for a single spike. Earth spike design is entirely outside the scope of this site, but note that in nice wet clay or loam soils a 1m x 13mm diameter earth spike will typically have an impedance around 100-120ohms ballpark, and worse in gravels or (worst) rocky areas. Double-up, use two or three linked together, and the result is still NFG as a protective earth`!

Here's why: If it were 'perfect', close to 0 ohms, the measured current would be correspondingly huge; V=IR and all that.

But now imagine this earth stake is collecting fault current from some sudden fault in your audio system; with your earth stake at, say, 25ohms - more typical for a group of 4 foot rods - the current that will flow will be ~10A at UK mains voltage. This IS NOT be enough to blow the faulty equipment's fuse, the basic intention of safety earthing. To blow a 13A fuse under 10s requires at least 26A and as much as 50A fault current. Consider the implications well - the equipment case is likely to remain live indefinitely, potentially deadly to the unknowing. This is why TT systems must use of RCDs set to trip at 30mA leakage and not fuses in the consumer unit.

(Oh, and UK plug-top fuses, equipment fuses, and breakers are a topic for another day in their own right.)

Finally, if you do disconnect the provided earth on a PME supply and substitute your own there is a real risk of electrocution during a fault in the rest of your house or, theoretically, those of your neighbours too. No wire or patch of earth has zero impedance, so fault current flowing to earth elsewhere will raise all bonded metalwork (think radiators and pipework) to a significant potential above your own earth spike. If, for some bizarre reason, you happened to be touching, say, the radiator while you fiddle with your hifi during such a fault there's a finite risk of electrocution. If you mistakenly happen to ground part of your kit to the radiator, whilst still relying on your own independent earth under such conditions, well you better budget for replacing at least the speakers... For information, 2 meters is considered sufficient separation between bonded items for certain classes of separately-derived earth bondings under IEE regulations - presumably because it would require a determined effort to bridge the gap!

However: in the UK you must NOT use incoming gas or water mains as an Earth bonding route. This is because the incoming main, and the Provider's service distribution that feeds it, is likely plastic and actually not formally earthed in and of itself as a service. These services are bonded to your local Earth reference within the home for your safety - 'equipotential bonding'. Read that again and think about it. Lashing your amp to the nearest radiator simplyt because it is metal and appears 'earthed' is a bad idea.

In passing, and here because it belongs nowhere else really, a caveat that applies to all PME system users: DO NOT connect earth and neutral together directly at any point, eg using a 'modified' plug, thinking that if they are bonded once, more must be better. The reason is fairly subtle: the earth wire exists as a separate wire inside your house for fault currents only - an escape path. If it was connected to the neutral it woud effectively become exposed to normal load current. If your neutral develops a fault, everything would continue to operate as normal except that all the items normally bonded to earth would float up to some indeterminate potential, possibly dangerously high, because the earth is now a service conductor. Think about that for a bit...

Note again that all this scary stuff refers only to PME systems. Where PME is not the norm locally, the rules are different again. If you have an electrical supply which is bonded to a Statutory Provider's Earth near the point it enters your house ('TT' - the norm in many non-suburban locations) there is no problem, and multiple earth spikes - correctly bonded only as required by your local electrical codes of practice - may indeed improve matters by lowering the Earth's apparent impedance. Multiple earth spikes should be spaced at least 2x length apart for lowest impedance. But the design and testing of TT Earth integrity systems is not the province of the DIYer - however well-intentioned.

We'll leave it there for now...just two things:

  • Play with your electricity supply and you are on your own. Big sparks, gods wrath and your insurers will likely come and meet you sooner rather than later. Do we need to say this ?
  • From 01 Jan 05 in the UK Electrical works such as this need to be signed-off by a suitably-qualified person or approved by your local Building control inspectorate, following the introduction of the new Approved Document Part P component of the Building Regulations.

    Further reading:

  • UK Building Regulations, Approved Documents Part P
  • Leaflet EMC 07: Protective multiple earthing on the RSGB Website, here: EMC Publications
  • Regulations for Electrical Installations (Edition 16) & lEE On-site Guide, available at good bookshops, your local library or via interlibrary loan.
  • BS7430: 2011 (Code of Practice for Earthing) Not likely to make a lot of sense to the interested amateur. It certainly couldn't care less about your soundstage issues.

    © the twisted pair 2002

  • 1.07.13: spurious crap removed. More bold red ink for idiots.
  • 8.02.10: couple of links updated
  • 3.01.07: Approved Document P link updated.
  • 04.09.02: Page rewritten in the light of better information...
  • 31.08.02: Page added