16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5

chapter 6

Installation control and protection
  3.1 - Introduction 3.5 - High temperature protection
  3.2 - Switching 3.6 - Overload currents
  3.3 - Isolation 3.7 - Protection from faults
3.4 - Electric shock protection

3.8 - Short circuit and overload
------- protection

3.8.6 -  Discrimination

Most installations include a number of protective devices in series, and they must operate correctly relative to each other if healthy circuits are not to be disconnect ed. Discrimination occurs when the protective device nearest to the fault operates, leaving all other circuits working normally.

Fig 3.26 System layout to explain discrimination

{Figure 3.26} shows an installation with a 100 A main fuse and a 30 A submain fuse feeding a distribution board containing 10 A fuses. If a fault occurs at point Z, the 100 A fuse will operate and the whole installation will be disconnected. If the fault is at X, the 10 A fuse should operate and not the 30 A or 100 A fuses. A fault at Y should operate the 30 A, and not the 100 A fuse. If this happens, the system has discriminated properly.

Lack of discrimination would occur if a fault at X caused operation of the 30 A or 100 A fuses, but not the 10 A fuse. This sounds impossible until we remember the time/current fuse characteristics explained in {3.6.3}. For example, {Fig 3.27} shows the superimposed characteristics of a 5 A semi-enclosed fuse and a 10 A miniature circuit breaker which we shall assume are connected in series.

If a fault current of 50 A flows, the fuse will operate in 0.56 s whilst the circuit breaker would take 24 s to open. Clearly the fuse will operate first and the devices have discriminated. However, if the fault current is 180 A, the circuit breaker will open in 0.016 s, well before the fuse would operate, which would take 0.12 s. In this case, there has been no discrimination.

To ensure discrimination is a very complicated matter, particularly where an installation includes a mixture of types of fuse, or of fuses and circuit breakers. Manufacturers' operating characteristics must be studied to ensure discrimination. As a rule of thumb where fuses or circuit breakers all of the same type are used, there should be a doubling of the rating as each step towards the supply is taken.

When fault current is high enough to result in operation of the protective device within 40 ms (two cycles of a 50 Hz supply), the simple consideration of characteristics as shown in Fig. 3.27 may not always result in correct discrimination and device manufacturers should be consulted.

When RCDs are connected in series, discrimination between them is also important, the rule here being that a trebling in rating applies with each step towards the supply (see also {8.6.3}).

Fig 3.27 To illustrate a lack of discrimination


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Extracted from The Electricians Guide Fifth Edition
by John Whitfield

Published by EPA Press Click Here to order your Copy.

Click here for list of abbreviations