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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. 

  • 0

    No you can't completely ignore Zs, but using an RCD it can be allowed to be much higher than if relying on overcurrent devices. E.g. to guarantee a 30mA device will trip in 0.4s you need at least 30mA to flow and hence max Zs needs to be below 230V/30mA = 7666Ω. (For a TT system with a 0.2s requirement, you'd be looking at needing 60mA and hence about 3.8kΩ). There's also a requirement to keep the touch voltage below 50V where the fault/leakage current is too low to guarantee tripping the RCD - so Ra < 1667Ω (which can be taken to mean Zs < 1667Ω if you don't have any more particular data).

    So yes, small excess values of Zs might be dealt with by adding a suitable RCD.

    Do check if Zs is reasonable for the circumstances - on a short circuit a high Zs may well be caused by corrosion or a loose connection - which means it can't be guaranteed, even to stay within the generous RCD limits. Also check that voltage drop limits aren't exceeded.

        - Andy. 

  • 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.

  • 0 in reply to AJJewsbury

    For reference the worst case Zs for RCDs can be found in BS 7671 in Table 41.5.

  • 0 in reply to Mike Mln

    Thank you very much

  • 0 in reply to AJJewsbury

    Thank you so much

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