to the 16th Edition IEE Regulations
   
   
   
 
 

chapter 5
Earthing

chapter 6
Circuits

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
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3.8.3 - Protection omitted

There are cases where a break in circuit current due to operation of a protective device may cause more danger than the overload or fault. For example, breaking the supply to a lifting electromagnet in a scrap yard will cause it to drop its load suddenly, possibly with dire consequences. If the field circuit of a dc motor is broken, the reduction in field flux may lead to a dangerous increase in speed. A current transformer has many more secondary than primary turns, so dangerously high voltages will occur~if the secondary circuit is broken.

In situations like these the installation of an overload alarm will give warning of the faulty circuit, which can be switched off for inspection when it is safe to do so. The possibility of short circuits in such cables will be reduced if they are given extra protection.

Probably the most usual case of omission of protection is at the incoming mains position of a small installation. Here, the supply fuse protects the installation tails and the consumer's unit. The unprotected equipment must, however, comply with the requirements for otherwise unprotected systems listed in {3.7.4}.

 

 

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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