to the 16th 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

3.3.3 Isolator positions

Every circuit must have its means of isolation, which must be lockable in the OFF position where remote from the equipment protected. The OFF position must be clearly marked in all cases so that there is no doubt in the mind of the operator as to whether his circuit is isolated and thus safe to work on. Adequate notices and labels must be displayed to ensure safe and proper isolation and to supplement locks to prevent inadvertent re-closing of isolators when this could cause danger. For single phase systems, both live conductors (phase and neutral) must be broken by the isolator (TT system). On three phase supplies, only the three phases (R, Y, and B or LI, L2 and L3) need to be broken, the neutral being left solidly connected (TN-S and TNC-S systems).

This neutral connection is usually through a link which can be removed for testing. Clearly, it is of great importance that the link is not removed during normal operation, so it must comply with one or both of the following requirements:

1. - it can only be removed by using tools, and/or

2. - it is accessible only to skilled persons.

In many circuits capacitors are connected across the load. There are many reasons for this, the most usual in industrial circuits being power factor correction. When the supply to such a circuit is switched off, the capacitor will often remain charged for a significant period, so that the isolated circuit may be able to deliver a severe shock to anyone touching it.

The Regulations require that a means of discharging such capacitors should be provided. This usually takes the form of discharge resistors {Fig 3.4}, which provide a path for discharge current. These resistors are connected directly across the supply, so give rise to a leakage current between live conductors. This current is reduced by using larger resistance values, but this increases the time taken for the capacitors to discharge to a safe potential difference. In practice, a happy medium is struck between these conflicting requirements, resistors with values of about 100K Ohms being common.

Fig 3.4 Discharge resistors connected to a three-phase capacitor bank

When isolating a circuit so that it can be worked on, care must be taken to ensure that there is no uninterruptible power supply (UPS) or other standby system which may make it dangerous although isolated from the mains supply.



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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