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

Question

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.

gkenyon · Answer

lyledunn said: method of limiting the volt drop along the cpc to 50v as referenced to the connection of the main protective bonding 
 This might sound very pedantic, and I mean no disrespect to lyledunn , but I feel it's important to correct a couple of points to avoid common misconceptions about supplementary protective bonding. 
 Using this method (Regulations 415.2 and 419.3), the voltage is not actually limited to 50 V for every possible fault. 
 It is limited to 50 V for currents up to and including the current which causes operation of the protective device (or, where multiple circuits share the same protective bonding, the largest of the protective devices) within 5 seconds. 
 Example: I have a B32 circuit-breaker to BS EN 60898. The current guaranteed to cause disconnection time within 5 s is 160 A, therefore supplementary protective bonding at the limit specified in Regulation 415.2 is guaranteed to limit to 50 V for fault currents up to 160 A. For fault currents exceeding 160 A, the touch current will exceed 50 V, but the circuit will be disconnected within 5 s. Certainly, where supplementary protective equipotential bonding is applied in cases where a disconnection time < 5 s cannot be met (e.g. 0.4 s), but a higher disconnection time of 1 s or 5 s could be met, the touch-voltage is not guaranteed to be limited to 50 V. 
 As Lyle points out, supplementary protective bonding is not really useful for B curve and C curve circuit-breakers, but is useful for fuses and D curve circuit-breakers. 
 lyledunn said: thereby allowing extension of disconnection times to 5s 
 This is also not the case. There is no maximum disconnection time to be applied for protection against electric shock where supplementary protective bonding is applied according to all of the requirements of BS 7671 (at least in "dry condition"). This is because, for currents below that causing 5 s disconnection time (i.e. leading to longer disconnection times), the touch-voltage does not exceed 50 V by virtue of the formula in Regulation 415.2. 
 Disconnection within a certain time for protection against overcurrent (in this case fault current) may, however, still be necessary dependent on the characteristics of the overcurrent protective device for the circuit, and the cross-sectional areas and materials of the line conductor and cpc.

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

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