The17th Edition of the Wiring Regulations gives us methods of fault protection within Part 4 Chapter 41 BS 7671.
But firstly what is Fault Protection?
Part 2 of BS 7671 Says…
“Protection against electric shock under single fault conditions”
In plain language this means that fault protection provides us with protection against receiving an electric shock from conductive parts that are not intended to be live but have become live due to a fault.
Methods of fault protection include…
- Earthing of circuits and equipment
- Protective equipotential bonding
- Automatic disconnection
Earthing Equipment and Circuits
We can be safe by providing an alternative low resistance pathway (CPC) for the electricity under fault conditions to flow. Earthing gives the fault current somewhere to go and can prevent you from becoming part of the circuit.
Earthing will not work if the current can flow through you more easily than through the earth conductor (CPC). This can happen when…
- Your equipment has a bad earth connection.
- You’re working in wet locations.
- You’re touching a metal object
What must be earthed?
- All low voltage circuits and extension leads.
- All noncurrent carrying metal parts.
- Portable & semi-portable tools and equipment unless double insulated.
Protective Equipotential Bonding
Main protective bonding conductors shall be connected from the Main Earthing Terminal of the supply to any extraneous conductive parts.
From Part 2 BS 7671 * Extraneous conductive part: A conductive part that is not part of the electrical system and liable to introduce a potential, generally earth potential, e.g. structural steelwork and metal gas and water pipes.
One of the greatest risks of electric shock is when we come into contact with a conductive part which has an earth potential of 0v and an exposed part that has become live at 230v due to a fault.
So to reduce the shock risk we make sure that all of the exposed or extraneous parts within a building are at the same voltage by ‘bonding’ (linking together everything that is ‘earthed’ within an installation).
This way, under fault conditions everything is at the same potential ‘230v’ and there should be no current flow if you become connected between an exposed conductive part that has become live at 230v and an extraneous conductive part which is now also at 230v as the potential difference is now at 0v between the two.
Remember this is only for a short period (usually less than 0.4 seconds) as the protective device should isolate the supply. Which is…
Automatic Disconnection
We must ensure that the protective devices are going to disconnect the supply within the times stipulated in Chapter 41 BS 7671 (see Table 41.1 for final circuits up to and including 32A).
This is done by ensuring that our measured values of Zs for any circuit protected by a circuit breaker to BS 60898 do not exceed 80% of the values within Tables 41.3 BS 7671.
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Remember if there is a failure of any of the three mentioned methods of shock protection we can provide ‘Additional Protection’ through the use of a 30mA RCD see regulation 415.1.1 in the 17th Edition of the Wiring Regulations.
If you’d like to know more take a look at our 17th Edition online course.