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Protective and protective Bonding conductor sizes

Udtwmc

I have a few questions on protective conductor and bonding conductor sizes, as I'm getting a bit hung up on terminology and other things!

Protective conductor [CPC]

My supply cable is 185sqmm 4-core (70C). Steel wire armour [k = 51] used as a CPC = 255sqmm 

my Zs [DB] measured is 0.0947 ohms (taken at low temperature say 20C). For CPC sizing using adiabatic eq. would I first apply the Cmax [1.1 * 230  or leave at just 230?] then divide by 0.0947 = 2.67kA (or 2.4kA no C max) ?

Do I take my disconnection time as what my CPD will actually operate in under earth fault conditions  (2.67kA = 0.08s) or what the circuit design disconnection time is: Ia = 5s  

so either 

S= Sqrt (2670^2 * 0.08) / 51 = 14.8sqmm round up to 16sqmm.  

or using 5s 

S= Sqrt (2670^2 * 5) / 51 = 117sqmm round upto 120sqmm ?

Additionally, I read in guidance note 8 [earthing and bonding)  a separate CPC for circuits greater than 160A is recommended which is true fin my case. so should this separate CPC be based on table 54.7 or the adiabatic eq.?

second part, main protective bonding size ...

should my main protective bonding circuit size "be not be less than half that 'required' for the CPC, not less than 6mm and need not be more than 25mm"  .... is this statement based on what the adiabatic eq. would deem acceptable eg so either 16sqmm for 0.08s disconnection time or 120sqmm for 5s disconnection time,  giving a bonding conductor size of either 10sqmm (not less than 16/2) or 25sqmm (based on 120sqmm) respectively

or should the bonding conductor be derived from the line conductor size of 185sqmm, therefore 25sqmm?

And finally, is the connection between the cable armour (terminated at the CW gland) to the main earthing terminal classed as a 'bonding conductor' so could potentially be only 10sqmm or should it be treated as a continuation of the actual CPC eg 95 or 120sqmm ? 

Thanks

 

  • +1

    as I'm getting a bit hung up on terminology and other things!

    :

    Protective conductor [CPC]

    Just to be clear, both c.p.c. and bonding conductors are both protective conductors - to give them their fill names protective bonding conductors (both main and supplementary) and circuit protective conductors - some parts of section 54 apply equally to both.

    Do I take my disconnection time as what my CPD will actually operate in under earth fault conditions  (2.67kA = 0.08s) or what the circuit design disconnection time is: Ia = 5s 

    Use the actual disconnection time given the fault current and the characteristics of the protective device (that's why we have all those charts in appendix 3).

    Do make sure you're considering the worst case in each situation though - e.g. for breaking fault currents worst case is probably with maximum fault current - i.e. minimum loop impedance - e.g. when conductors are cold, while for ensuring ADS worst case is probably with lowest possible fault current, highest impedance, so when the conductors are hot. So some adjustment of R1+R2 might be needed.

       - Andy.

  • 0
    Udtwmc said:
    my Zs [DB] measured is 0.0947 ohms (taken at low temperature say 20C). For CPC sizing using adiabatic eq. would I first apply the Cmax [1.1 * 230  or leave at just 230?] then divide by 0.0947 = 2.67kA (or 2.4kA no C max) ?

    I guess you have to start with some reliable premise but I wonder just how accurate your measured value is. Even very slight adjustments in that value can lead to significant differences in Ipef. 
    The accuracy of the instrument at those relatively low values could be questionable. Further, the value will not account for the reactive aspect and Ze actual might be a tad higher.

    If your CPD is a circuit breaker then let through data should be readily available. This can then be compared with the thermal capacity of the cable. When considering instantaneous operation of a circuit breaker, the most onerous energy let-through is when current is highest so using 1.1*230 with cold cable would seem appropriate.

    When the CPD is a fuse then the opposite is mostly true. Use 0.95*230 with cable at operating temperature.

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