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RLV systems exceeding Zs

E95

Hi,

First question - Does T41.1 (0.8s for TN) exclude disconnection time for RLV systems (411.8.3) which states "The EFLI at every point of utilisation shall be such that the disconnection time does not exceed 5s", so satisfy T41.6 or 411.5.3 by using an RCBO or RCD

Second question - To risk assess a high EFLI on a RLV socket circuit would you take into account:

Risk of fire due to L-E or L-N fault

L-E fault

  1. Class II equipment, no possibility for L-E short within equipment
  2. Cabling PAT tested etc.
  3. Fire detection and suppression systems in place

L-N fault

  1. Zs not related to L-N fault provided that R1 value is acceptable
  2. If formula for (K^2 * S^2)/(I^2) is satisfied

Any other points to consider whether for or against accepting a circuit exceeding maximum Zs?

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  • 0

    If by RLV you are referring to the split phase 55-0-55 supplies found on building sites etc then when they were devised the safety case was intended not to need such reliable ADS - the maximum voltage exposed to ground is 55v (though maybe higher in some 3  phase derived systems if the two lives are not from the same phase) In any significant fault the exposed voltage is pulled below 50V then in principle from a safety of life point of view ,then there is no need to disconnect ever - there is a reason that RCD currents and rod resistances are coordinated around a 50V rise ;-)  The ' turn off in half a heartbeat or fibrillation starts' consideration that drives the safe-ish ADS times in a normal 230V TN system does not really apply. So we are left with damage or injury from things getting hot. Hence the 5 seconds, not 0.4 or 0.8 or something. It was also agreed that there would not be a distributed neutral, and all loads would be wired live to live.

    Since all that was decided attitudes have changed, and RCDs are now de-riguer,  and appear on these 55-0-55 systems even though the actual fatal shock risk is much lower. An RCD is likely to be much faster than you need anyway.

    Mike.

  • 0 in reply to mapj1
    mapj1 said:
    Since all that was decided attitudes have changed, and RCDs are now de-riguer,  and appear on these 55-0-55 systems even though the actual fatal shock risk is much lower

    There has never been a fatal electric shock on such systems since their introduction (Paul Cook Commentary on the Wiring Regulations 17th Edition)

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  • 0 in reply to mapj1
    mapj1 said:
    Since all that was decided attitudes have changed, and RCDs are now de-riguer,  and appear on these 55-0-55 systems even though the actual fatal shock risk is much lower

    There has never been a fatal electric shock on such systems since their introduction (Paul Cook Commentary on the Wiring Regulations 17th Edition)

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