How to Create a Clean Earth connection for Power Conditioning

Earthing power conditioners - 4 options

The earthing connections for conditioners installed to isolate noise are very important.

The unit is designed to isolate the load from the building earth as well as to provide voltage stabilisation and noise attenuation.

The earth on the protected load may be connected to the safety earth but it must be realised that noise spikes on the safety earth may cause problems.

This applies to so-called 'clean' or 'dedicated' earths run from the main building power distribution box.

Ideal installations have the input of the conditioner protected by the safety earth and a separate clean earth provided for the critical load.

This can be achieved by an earth rod, water pipe (when suitably tested), or structure in tall buildings.

A 'clean' earth may be made by employing any of the following:

Earth rod

Most installations can be earthed by driving a 10 mm x 2.5 m long earth rod within 30 m of the critical load.

This rod is then connected using at least the same size wire as the feeder circuit.

Water pipe

A water pipe can be used if the resistance to earth is less than 6 Ohms.

Make certain by testing that the water pipe is not isolated by plastic connections, especially where it leaves the building.

Building structures

When the installation is in a multistorey building it will be necessary to connect to the structural steel for the clean earth.

Choose a point that is close to the conditioner and bolt a wire to the structure. Connections to the structure are usually better than running a wire to the basement.

Earth usage

In those cases where the load is partially protected (some peripherals unprotected by the conditioner) we recommend that the low or pseudo neutral side of the conditioner output is connected to the clean earth to avoid voltages appearing between the floating output of the double wound CVT and the unprotected peripheral.

Further attention should be given to ensure that earth connections are star-wired to the clean earth.

Local permanent wiring regulations should always be observed.

The four major options are as follows:

1 the output of the conditioner is floating with the safety earth wired through. Advance plug/socket units are like this. Unit fails normal earth loop impedance testing but is safe to BS 3535. If one output is fault connected to earth the other becomes hazardous. The conditioner will work OK. If both outputs are earthed the unit will close down to a safe condition. Note that some poorly designed SMPS are sensitive to floating neutrals.
2 hardwired using separate clean earth. This gives good overall noise performance. Clean earth must test out properly and be isolated from safety earth. Clean earth should be labelled in UK to 514-7.
3 as 2 but establishment of quasi neutral. Output high is now potentially hazardous and should be connected via an RCCD if feeding sockets. Common mode noise may be better than 2 above in some situations. For first time installations this option gives the most straightforward solution. The conditioner operates like a new distribution transformer.
4 safety earth wired through quasi neutral. Common mode attenuation poorer. Installation should use RCCD with sockets.

The output is high-and-low not live-and-neutral so earthing the low output terminal is entirely compatible with the Regulations.


Many electrical systems include an earth, but not all, this page simply sets out the different types of earth used in Power Conditioning and in particular tries to define a 'Clean Earth'.

Definition of Earthing

A circuit point or a connection line can be said to be correctly earthed if, within the frequency range under consideration, the impedance (Z) between this point and earth is lower than 5 Ohms.

Definition of 'Clean Earth'

A low impedance Earth point with little or no chance of conducted noise either already present or likely to be created when bonded to a circuit

Earths are used for two main reasons

1. Safety

An electrical shock comes from placing a part of your body in such a way as to complete an elctrical circuit, since current can't flow without a complete circuit there are two main differing approaches to minimising the risk of electrical shock.

Definition Pros Cons
Isolated systems Both 'supply' and 'return' wires are isolated from earth One Fault tolerant
Shock only results from touching both supply and return
Can be distributed as two wire
Safety earth has to be locally provided
Earthed Systems One wire (Neutral) bonded directly to Earth Use of RCD can protect against shocks
Neutral wire nominally safe
Can be distributed as two or three wire
No Fault tolerance
Shock from touching live if standing on Earth

UK domestic applications are all earthed, the most common system being used is Protective Multiple Earthing (PME). In PME systems a typical mains incomer consists of two or three cores (live and neutral with or without a separate earth), modern installations use metal shielded cables where the shield is also earthed. At the Distribution point of the House all of the earth potentials are bonded together on a main Earth bus bar and (if available) a further link is taken to any incoming water or gas pipes that are metallic and emerge from underground. In this way many routes to earth are available and the impedance of the earth is likely to be low even if one link fails. Industrial applications are basically the same although three phase supplies can be in several different configurations in the UK, in all of these the Neutral is bonded to earth at the sub-station.

2. Zero Reference

A large proportion of electrical devices contain some form of electriconic component many of which require a reference 'zero' or 'ground' for optimal operation. Noise (in the form of AC or DC signals) on this zero reference can lead to unexpected results including failure of the electronics.

The reasons for generating a Clean Earth

Because the Earth is multiple bonded and common to all systems connected to the supply it is possible for it to become 'contaminated' with signals. Some equipment is particularly bad at generating common mode noise others are particularly sensitive to it. Effective isolation of either the source or the sensitive system is an effective way of reducing the problem. A complete solution is by BOTH isolation and the use of a Clean Earth.

Local Earthing points that may be suitable for a Clean Earth

Structural steel, metallic water pipes and specially positioned Earth rods and mats

A copper earth rod showing threaded tip and clamp point A copper earth mat A copper earth signal reference ground

Only testing of the Earth Impedance can determine wether your clean earth is suitable for use

The last of these the 'Signal Reference Ground (SRG)' serves a dual purpose, as well as providing a suitable safety earth protection it also offers a certain amount of protection against Earth Potential Rise (EPR) faults. Certain circumstances (most noticeably lightning strike) can raise the potential of earth locally and do so over a large area with a fall in potential further from the strike. In a multiple earthed system this could mean the 'earth' at one end of the room seeing a significantly higher potential than the 'earth' at the other end and a because the wiring resistance is considerably lower than the earths resistance a fault current could flow, damaging anything in it's path.

The fitting of a SRG provides a safe path for the fault current in preference to local wiring.

A = Conductors are typically 50 mm wide x 0.40 mm thickness (#26 AWG) copper strip

B = SRG welded connections

C = Welded connection to pedestal, #6 AWG (4.7 mm)

D = Welded connection to SRG, #6 AWG (4.7 mm)

E = Low impedance riser equipment bond

F = Welded connection, low impedance risers to SRG

G = Power centre ground, #4 AWG (5.9 mm)

H = Welded connection to steel column, #4 AWG (5.9 mm)

J = Pre-engineered sections are up to 16 ft (4.8 m) wide

K = Welded connection to steel column, #6 AWG (4.7 mm)


Effective Power Conditioning is from the use of an isolated Live, Neutral and a Clean Earth.