Discuss Fault current, and earth electrode resistance: Numbers not making sense in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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J

Jodes

In "Advanced Electrical Installation Work" by Trevor Linsley, he says that an earth electrode rod that is 1m deep will typically have a resistance of between 10 and 200 ohms. Given that fault currents can supposedly, momentarily, be in the order of 1000's of amps, this makes me scratch my head:

Let's say 1000 amps and 10 ohms, that would be a p.d. of 10kV. But given a 240v supply, the highest fault current possible would be 24A. So a short between live and earth would bring just about every bit of metal up to 240v, potentially without a single fuse blowing!! (Ignoring RCDs)

What am I missing?
 
you're not missing anything. on a TT earthing system the Ra (and Ze) is generally too high to allow a fault current that will trip an OCPD, therefore a RCD is used for fault protection.
 
In "Advanced Electrical Installation Work" by Trevor Linsley, he says that an earth electrode rod that is 1m deep will typically have a resistance of between 10 and 200 ohms.

Well for a kick off, an earth rod that is only 1m deep is the next best thing to useless, and would never achieve anywhere near a 10 ohm value, it would also not even be a stable installation. As an absolute minimum, in decent soil type conditions an earth rod position should be made up of at least two 1.2m 5/8''-3/4'' rods coupled together (2.4m depth) or a straight 2.4m 5/8-3/4'''' rod. Your basically aiming for the lowest Ra value you can achieve and certainly not this numpty 200 ohms that's often banded about. A good degree of system stability will also be achieved. Depth is the overwhelming factor to achieving both stability and low Ra values, the deeper you go the better. Couple electrode depth with soil conditioning or better still electrode enhancers such as Bentonite and Marconite and you're well on your way to creating a working TT system!!
 

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