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TN system - Max Zs Vs tripping time in the Presence of RCD protection

Pabindu Gamage

Hi, 
I am trying to confirm a point. 

as long as we meet the max disconnection time (let say 0.4s for a TN system) . for a small power circuit, 
Can we ignore the Max Zs requirement ? 

Example 
Small power  radial circuit ,20A RCBO 30mA , type A C curve, 
RCD trip time is 20ms 

but, max Zs is higher than it should be to Overload trip before 0.4s (lets say Max allowed  Zs is 1.09 but the measured is 1.40)

Can we override the 0.4s overload tripping requirement as its already being met with RCD protection of the RCBO ? i.e. as it trips at 20ms anyway so it meets the 0.4s requirement. 

Or is this a subjective matter? 

Thank you all in advance. 

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  • 0

    RCD trip time is 20ms 

    Just to add that test results can't really be used for calculations - just because a device tripped in 20ms a week last Wednesday in warm dry weather doesn't mean it'll necessarily do the same under different conditions (cold, damp, the device 5 years older and not been exercised for ages) - all the test really tells you is that it's likely to be working within specification - so you still need to use the general data for calculations - i.e. 300ms at 30mA, 150ms at 60mA and 40ms at 150 or 250mA.

      - Andy.

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  • 0

    RCD trip time is 20ms 

    Just to add that test results can't really be used for calculations - just because a device tripped in 20ms a week last Wednesday in warm dry weather doesn't mean it'll necessarily do the same under different conditions (cold, damp, the device 5 years older and not been exercised for ages) - all the test really tells you is that it's likely to be working within specification - so you still need to use the general data for calculations - i.e. 300ms at 30mA, 150ms at 60mA and 40ms at 150 or 250mA.

      - Andy.

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