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TN-S - Additional PE Conductor

Nick Parker

Why in TN-S System PE Cables have to be provided in addition to the phase conductors for each loads (like socket, light fixture, Motor etc.)?
Don't bonding the enclosure of the loads (socket, fixtures, Motors etc.) to the local earth electrode or grid makes it sufficient and the fault current takes the route of enclosure --> earth electrode / grid --> return via mother earth to the source?

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    Earthing to a local electrode is a TT system - the resistance of the soil around the electrode is often in region of tens to hundreds of Ohms - with the result that earth fault currents are often in the region of 1 to 10A - not enough to blow even a 5A fuse or trip 6A MCB quickly. Thus typically RCDs are needed for automatic disconnection of earth faults in TT systems.

    In TN systems including TN-S, local earthing is provided by solid metallic connections back to the supply's Earthed point point - thus making Earth fault currents much larger - hundreds or thousands of amps - in this way ordinary fuses and circuit breakers can reliably open quickly on Earth faults without the complexity, cost or reliability issues of having to rely completely on RCDs and local electrodes.

       - Andy.

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    Earthing to a local electrode is a TT system - the resistance of the soil around the electrode is often in region of tens to hundreds of Ohms - with the result that earth fault currents are often in the region of 1 to 10A - not enough to blow even a 5A fuse or trip 6A MCB quickly. Thus typically RCDs are needed for automatic disconnection of earth faults in TT systems.

    In TN systems including TN-S, local earthing is provided by solid metallic connections back to the supply's Earthed point point - thus making Earth fault currents much larger - hundreds or thousands of amps - in this way ordinary fuses and circuit breakers can reliably open quickly on Earth faults without the complexity, cost or reliability issues of having to rely completely on RCDs and local electrodes.

       - Andy.

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