From the consumer’s point of view another important parameter of the supply system is its impedance as viewed from his terminals. On the one hand, the lower the impedance the greater will be the stress on his switchgear and protective devices, but on the other hand, the higher the impedance the greater will be the risk of annoyance due to distortion caused by either the consumer’s own load or by that of a nearby consumer.

High network impedances are troublesome to installation designers because they result in low values of fault current, which severely limit the number of series graded protection devices and cause an increase in the I2t energy let through of inverse characteristic devices such as fuses.

The 16th edition of the IEE Wiring Regulations, BS 7671, requires installation designers to have a knowledge of the limits of system impedance to which the supply will be kept in order that they may install the necessary protective devices to an appropriate rating and to operate within the required time.

All supply systems are dynamic and many DNO staff are continually employed laying cables and moving and installing plant in order to ensure that the system configuration meets the demands of the customers. For this reason it is not possible to give an exact impedance figure for any one location, but the appropriate local area office should be able to give installation designers the maximum and minimum likely to be encountered for a particular location.

A maximum earth loop impedance figure of 0.35W is quoted nationally for l.v., singlephase, PME system supplies of 100 A or less. An appropriate maximum prospective short-circuit current of 16 kA is quoted for many urban supplies; further information may be gained from Engineering Recommendation P25/1.

For many years it has been common practice to express the energy available on short-circuit in terms of ‘short-circuit MVA’. This is simply where V is the normal system voltage between phases and A the symmetrical component of the short-circuit current.

In 1971 the International Electrical Commission (IEC) introduced a standard for switchgear ratings (IEC 56) which specified that the working voltage rating should be expressed in terms of the system maximum, for example 12 kV for an 11kV system, and that short-circuit ratings should be expressed in terms of the maximum symmetrical fault current. A range of ratings was specified, for example 12.5, 16, and 25kA for 12kV gear.

For any three-phase system voltage the short-circuit level and the system impedance are inverse functions of each other, . On l.v. systems the cables will generally be the major contributors to the system impedance as the h.v./l.v. transformers are of low impedance. The governing factor is thus the distance of the consumer from the nearest substation.

On DNO h.v. systems the short-circuit ratings of the switchgear have a considerable economic significance and, therefore, system designers aim at keeping these to as low a figure as practicable. A common method of achieving this is to employ high. From the consumer’s point of view another important parameter of the supply system is its impedance as viewed from his terminals.

On the one hand, the lowerthe impedance the greater will be the stress on his switchgear and protective devices, but on the other hand, the higher the impedance the greater will be the risk of annoyance due to distortion caused by either the consumer’s own load or by that of a nearby consumer.

For any three-phase system voltage the short-circuit level and the system impedance are inverse functions of each other, . On l.v. systems the cables will generally be the major contributors to the system impedance as the h.v./l.v. transformers are of low impedance.

The governing factor is thus the distance of the consumer from the nearest substation. The high impedance is achieved by judicious spacing of the windings and does not increase the transformer losses or costs to any appreciable extent.

The high impedance does not affect the voltage output as the tap-changer regulates accordingly. At 11kV the impedance of the cables is generally much less significant. Until the publication of IEC 56 many 11kV systems were designed for a maximum level of 250MVA which is equal to 13.1kA.

The new rating method therefore poses a problem where new or additional switchgear is required on an existing system, since the 16 kA switchgear is more expensive. In general, however, UK manufacturers can supply switchgear tested to 13.1 kA at similar to 12.5kA prices.With the low growth of demand, these 250mVA systems will remain for many years.

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