A copper earth rod showing threaded tip and clamp point
A copper earth mat
A copper earth signal reference ground
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".
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.
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
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 |
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| Isolated systems |
Both "supply" and "return" wires are isolated from earth |
One Fault tolerant |
Safety earth has to be locally provided |
| Earthed Systems |
One wire (Neutral) bonded directly to Earth |
Use of RCD can protect against
shocks |
No Fault tolerance |
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.
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.
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.
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 |
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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 both an isolated Live and Neutral and a Clean Earth.
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