• Supply earth
  
• Earth pin of a mains plug
  
• Common negative
  
• Antenna earth system (eg, a set of buried radials)
  
• Distribution earth
  
• Surge earth
  
• Intrinsic safety earth
  
• Busbar in a panel or cubicle
  
• Test equipment front panel (eg, oscilloscope)
  
• A conductive rod driven into the ground
  

• All equipment is metal-cased.
  
• All equipment sits directly on a metal sheet to which it is electrically bonded. Everything shares the same low-impedance zero volt reference.
  
• For good measure, the metal 'earth plane' is at ground level and connected to ground by a system of rods driven into the soil so that it is at local ground potential.
  
• There is no connection to other electronic systems.
  
• The system is physically small, a few square metres at most, so making the likelihood of a direct strike negligible.

The purpose of the metal 'earth plane' sheet is to provide a low impedance to any induced currents which flow, resulting in very small induced voltages. Such an area of zero or minimal potential differences is sometimes referred to as an 'equipotential zone.'

It is worth explaining what is meant by 'low impedance'. This is a loose term and the meaning depends a great deal on the context and application. With lightning-induced surges, currents of the order of 1 kiloamp may be involved, developing a potential difference of 1 kilovolt across each ohm of impedance for each kiloamp of current which flows. The lowest impedance is provided by a sheet of 'high-conductivity' metal, which, strictly, should be non-ferrous because of the skin effect which is more pronounced in ferro-magnetic alloys and which forces transient or high-frequency currents to flow through smaller conductor areas. For a 50-60Hz electrical supply, the purpose of the protective earth conductor is to provide a ‘low impedance’ to supply frequency fault currents, so that voltages developed across a length of cable are insufficient to cause a serious electric shock to any people within the installation. Metal plumbing and heating pipes are bonded to the protective earth conductor system to create a safe equipotential zone. However, as we shall see, this system, though adequate at the supply frequency, cannot be considered a low impedance equipotential zone for lightning-induced transients.

Now that we have a system of cables, we do need to worry about lightning-induced transients. So let's evaluate in more detail the way in which cables can pass transients to our equipment and the means by which surge protection devices operate.

Before a surge can damage electronic equipment, several conditions need to be fulfilled. First, sufficient voltage must be present between two vulnerable points on the equipment. The vulnerable points are usually signal or power supply inputs or outputs, and the equipment’s zero voltage reference point (commonly, the casing or chassis connected to the mains supply earth). The voltage above which significant current starts to flow is often called the breakdown voltage (or potential). The current must flow for sufficient time to deposit enough energy within electronic components to cause damage, eg, the melting down of some part of the device.

It is essential to protect all cables which introduce a significant risk. Consider again our system on its earth plane, but now with all incoming cables feeding equipment through suitable SPDs. Because of the low impedance earth plane, this will still be close to an ideal system.

Real-world Earthing In reality, very few systems are bonded to an earth plane or mat, as is shown above. Consequently, a thorough knowledge of real-world earthing is necessary, if the deployment of surge protection devices is to succeed.

In the real world, earthing is done with cables and similar conductors. The use of separate connections to ground, such as separate ground rods, can cause unwanted voltages to be developed across the ground impedance (see figure left). These can be appreciable. For example, a 100A peak surge current flowing through a 10 ohm ground impedance will develop 1000V through the ground. Damage to the equipment is inevitable!

The right hand figure illustrates an SPD connected between an incoming signal line, shown as a wire pair, and a piece of equipment. The SPD is connected to the equipment earth conductor (typically the protective earth conductor) which, in turn, is ultimately connected to ground. This is an improvement over the previous figure but it gives a relatively long path that introduces impedances and hence unwanted voltages to the protected equipment.

To avoid the impedances and potentially damaging voltages associated with the previous configuration, re-position the earth connection, as shown in the left-hand figure. This has the effect of locating the equipment earth (ie, its zero-volt reference) at the SPD earth point. There is still a large transient voltage developing between the ground point and the SPD, but this not appear across the equipment which receives only the limiting voltage of the SPD - which is as it should be.

A typical installation includes a number of items of equipment or devices, each with its own connection. If these are all connected to a common point, from which a cable runs to ground, the result is a star-point earth system. Additional important details on star-point earthing and related topics are available in the full-length version of this document (Telematic Application Note 1003 - An Earthing Guide for Surge Protection). Telematic consultants are available for on-site SPD design assistance throughout the world.              Return