Earthing Guide for Surge Protection Device for Homes
In this article, we discuss briefly How to Connect Earth to SPD and where to place SPD at your home. this complete article is based on Earthing Guide for Surge Protection Devices by Eaton. Eaton is an American-Irish multinational power management company founded in the United States. Eaton has more than 86,000 employees and sells products to customers in more than 175 countries.
What is Lightning?
A lightning flash is caused by an electrical current flowing into the atmosphere. Moist air currents interacting with ice particles within a cloud lead to the formation of concentrations of electric charges at different heights. Very large voltage differences, of the order of many millions of volts, develop between the charge concentrations and the base of the cloud and the surface of the earth. When this voltage difference becomes sufficient to overcome atmospheric resistance, a lightning stroke occurs. Most lightning strokes take place cloud-to-cloud but some are cloud-to-ground. In the UK, it is believed that 98% of direct cloud-to-ground strokes carry a current of 200kA or less, with a median of around 30kA. For these direct Lightning strikes make sure the building housing the equipment is provided with structural lightning protection in accordance with national standards.
SPD is used to protect against indirect surges coming from external cables like Mains power, Telephone, Cable TV, Dish Antennas, Computer network or LAN, External lighting power cables, etc.
How SPD works?
Surge protection devices limit the transient voltage to a level that is safe for the equipment they protect by conducting the large
surge current safely to the ground through the earth conductor system. Current flows past, rather than through, the protected equipment and the SPD thereby diverts the surge. The SPD limits both common and difference mode voltages to the equipment. The voltage which the equipment receives during a surge is called the ‘limiting’ or ‘let-through’ voltage One way of regarding a surge protection device is as an earth connection which is only present during a surge.
The problem of Use of Separate Connections to the Ground
The use of separate connections to the ground, such as separate ground rods, can cause unwanted voltages to be developed across the ground impedance (figure 11). These can be really high volatge.
Assume an SPD with a limiting voltage of 16V is connected as shown in figure 12 and that the impedance from the body of the SPD to the equipment earth, via the ground, is 10ž. Depending upon the soil and other circumstances, this can be a credible low value. A 100A peak surge current flowing through the ground impedance will develop 1000V through the ground. The SPD will limit the voltage across itself to 16V and the equipment will be subjected to 1016V instead of the required 16V. This will cause Damage to the equipment.
Trouble High Impedance Surge Earth
The problem of having high surge earth inductance is due to the long distance between SPD and the earth pit. SPD is placed in DB away from the earth pit and is connected between an incoming 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 the ground. This gives a relatively long path.
The cables can be represented by their equivalent impedances – inductance in series with resistance – as shown in the figure.
A common mode surge appearing on the cable indicates the presence of a transient voltage between the cable and the ground connection. The SPD operates and a rapidly increasing current starts to flow down the surge earth. The voltage across the SPD is limited to its normal limiting voltage. However, due to the relatively high impedance of the surge earth, a large voltage appears across it. The equipment sees this voltage plus the SPD limiting voltage – and the transient voltages can be added to the diagram while if there is a breakdown (usually destructive) within the equipment, another current path is generated. Note that current now flows out through the ‘protected’ or ‘safe’ end of the SPD. If the current path has a low enough impedance, the equipment, and the SPD output components can be damaged.
The Best Position for Spd and Earth Connection to Reduce the Limiting Voltage
The limiting voltage can be lowered by re-positioning the earth connection. Locating the equipment earth (i.e. its zero-volt reference) at the SPD earth point produces the configuration shown in figure 19. Placing Type 1 & 2 SPD in energy meter box where power cable enters the house and at the same place is the nearest point to earth pit.
The equipment is now subjected to an inductive voltage transient across the earth cable. There is still a large transient voltage developing between the ground point and the SPD, but this does not appear across the equipment which receives only the limiting voltage of the SPD – which is as it should be.
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