Q & A of the Day - Is it better to touch the instrument probe on the busbar?

Question: This is, perhaps, an unusual question - concerning earth loop impedance measurements - when we carry out a Ze measurement at the origin, is it better (or more accurate) to touch the instrument probe on the busbar, rather than on the line terminal of the main switch?

Should we instead, test between the disconnected earth and the end of the busbar in order to include the resistance of the busbar right up to the circuit breakers/just before the start of the final circuits)?

I ask this because Zs = Ze+(R1+R2). I would imagine that a 'resistance gap' is being neglected between the main switch line terminal and the start of the final circuits if we test the conventional way, as the resistance of the busbar is being omitted.

Answer: The Ze measurement should be taken as near as possible to the origin of the installation. If taken after the breakers, eg: at the outgoing side, some small value of resistance may be included through the breaker.

The Zs=Ze +(R1+R2) equation is an approximation and is seldom accurately measured on site.

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