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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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    As per Andy's answer what you describe is done, it is just not called TNS, it is TT. 

    The name TNS is reserved for the case where there is a dedicated metallic connection back to base all the way.

    Now TT is common, especially in places where the earth connection may be hard to maintain, and a break may be un-noticed, such as overhead feeds and large rambling places like farms.

    However, TT has its problems, the electrodes have to be maintained, and in some soil types that free drain - gravels and sands can be fun, getting a good connection can be problematic - if the resistance between your electrode and the one at the substation is hundred ohms or more (and that is quite common in some places ) then it is not going to blow even a small fuse if there is a fault. Then additional safety measures must be taken, usually RCDs. For the same reason during a fault the earth electrode and every earthed item in the installation rises to a high voltage until the fault current is interrupted. 

    In contrast a well built TNS system has a large fault current but low touch potentials, there are cases when that is a good thing, and others when it is not.

    Mike.

     

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  • 0

    As per Andy's answer what you describe is done, it is just not called TNS, it is TT. 

    The name TNS is reserved for the case where there is a dedicated metallic connection back to base all the way.

    Now TT is common, especially in places where the earth connection may be hard to maintain, and a break may be un-noticed, such as overhead feeds and large rambling places like farms.

    However, TT has its problems, the electrodes have to be maintained, and in some soil types that free drain - gravels and sands can be fun, getting a good connection can be problematic - if the resistance between your electrode and the one at the substation is hundred ohms or more (and that is quite common in some places ) then it is not going to blow even a small fuse if there is a fault. Then additional safety measures must be taken, usually RCDs. For the same reason during a fault the earth electrode and every earthed item in the installation rises to a high voltage until the fault current is interrupted. 

    In contrast a well built TNS system has a large fault current but low touch potentials, there are cases when that is a good thing, and others when it is not.

    Mike.

     

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