This question concerns Ze (earth loop impedance external to installation) readings on a 32A TNS electrical installation. It is answered by SELECT (Theme - Testing, Tools and Equipment):
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Question: I have recently come across a TNS electrical installation that showed a Ze reading of 0.7. I know that this is acceptable as the maximum allowed is 0.8, but it made the ring main's (32A 60898 Type B) Zs reading come in at 1.4. Is this a fail? Or am I using the wrong table?
Answer: You need to compare the measured Zs values against the tables in the regulations - normally Table 41.3 where a maximum value for a 32A Type B circuit breaker is 1.44.
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