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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?

Parents
  • 0

    First question - Yes, 5 s is the disconnection time, use Table 41.6 where OCPDs are used for ADS, or effectively Table 41.5 where RCDs provide fault protection (see 411.8.3 and page 40 of the 2022 edition of Guidance Note 5 Protection against electric shock). Need to make sure the RCD (and particularly its test button) are rated for the RLVS system.

    Second question - Not sure why we are risk assessing? It's a requirement of BS 7671 to be able to achieve 5 s disconnection time, AND meet the requirements for protection against overcurrent for all possible faults of negligible impedance, e.g. for single-phase that would be L1-L2, L-PE and L2-PE.

  • 0 in reply to gkenyon

    Thanks Gary, 

    The challenge has came to evaluate the risk since the owners simply don't want to isolate (Which I disagree with), however they're coming back with class II appliances etc. Which regardless won't protect against a L1-L2 as a stop gap until double pole RCBOs can be sourced, the phrase used by the decision makers was "Low likelihood of fault in a short period of time"

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  • 0 in reply to gkenyon

    Thanks Gary, 

    The challenge has came to evaluate the risk since the owners simply don't want to isolate (Which I disagree with), however they're coming back with class II appliances etc. Which regardless won't protect against a L1-L2 as a stop gap until double pole RCBOs can be sourced, the phrase used by the decision makers was "Low likelihood of fault in a short period of time"

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