Fault ratings of cables

[davva]

When you size a cable for fault rating do you use the "asymmetrical" or "symmetrical rms" fault current at the point of concern in the network.

I realise it is the I^2xt heating effect that is used in the standard adiabatic formula for sizing the cable. The formula is relevant for faults between 0.2secs and 5secs, so is it taken that the DC offet current has decayed to zero by this time (0.2secs). What happens below 0.2secs? I had convinced myself I knew the answer but now I'm not so sure!!

I would also be interested in hearing from people who have any experience in "undersizing" cables, i.e. selecting a cable which would exceed the "recommended" final conductor temperature for a given fault current and disconnection time, e.g. taking a conductor (rated for 250degC) to 300degC. Would it catch fire? Would the insulation become brittle? The intention would be to replace the cable after the short circuit had occured, i.e. it would not see repeated short circuits. (The cable would be suitably braced for electromechanical forces). I guess what I'm also asking is when they say final conductor temperature do they mean final or is there any headroom?

 

[AusLee]

The method of symmetrical components gives the highest short circuit value and it is considered as over dimensionning since 95% of the sc cases are asymetrical.

However, you can still give an acceptable answer if you calculate as per IEC 60909-1 "Short-circuit currents in three-phase a.c. systems".

I²t is the only criteria. You can "safely undersize" the cable is you make sure that the circuit breaker will trip before the time it takes the cable to reach 300C under the calculated fault current. You can also "undersize" the fuse to blow before the cable. Like you said: the cable must be braced.

"Final" means: the temperature the cable may reach, but not exceed, and return to safe and normal operations after the fault is cleared.

The headroom is in the equation: Heat Absrobed = (T_final-T_initial)*massic_heat_absorption_coefficient (per meter length), so 300C over 1ms is tolerated better than 255C over 2 seconds.

In my opinion, s.c. protection is neither for the cable nor for the breaker: it is there to protect your expensive servers, metal halide lamps, motors and the like.

 

[cuky2000]

Several answer to your questions may be find in the enclose graph and comments.

 

[davva]

Thanks to AusLee and Cuky2000 for the advice and charts I will look up this IEEE std for further information and wipe the dust of my thermodynamics book to check out that equation!!

 

[cflatters]

As stated, the formulae takes into account the initial assumed (conductor operating) temperature and the final limiting temperature in the form of a 'K' factor. These differ according to conductor and insulation.

The system design should satisfy the basic adiabatic equation

K^2S^2 >= I^2t

Manufacturers provide this in the technical cable tables.

BS 7671 Includes K factors for UK installation, IEC 60364 applies to internaltional projects.