Protection of cables in accordance with the 16th Edition of the IEE Wiring Regulations (BS 7671).

Protection Against Overcurrent
Overcurrent is defined in the 16th Edition of the Wiring Regulations as "a current exceeding the rated value. For conductors the rated value is the current-carrying capacity." Overcurrent can be divided into two individual levels of fault these being overload current and short circuit current. These should be considered separately.

Protection Against Overload
Overload is defined in the 16th Edition of the IEE Wiring Regulations as "an over current occurring in a circuit which is electrically sound". This may be the result of too many appliances drawing current from a system, a faulty appliance, or a motor subjected to mechanical overload.

Regulation 433-01-01 of the 16th Edition of the IEE Wiring Regulations defines the basic requirement for overload protection, "protective devices shall be provided to break an overload current flowing in the circuit conductors before such a current could cause a temperature rise detrimental to insulation, joints, terminations, or the surroundings of the conductors. Circuits shall be so designed that a small overload of long duration is unlikely to occur".

Co-ordination Between Conductors and Protective Devices
It is apparent that Regulation 433-01-01 of the 16th Edition places emphasis on the surroundings of the conductor as well as the conductor itself. Regulation 433-02-01 has laid down three conditions to meet this requirement:

a) IbIn
b) InIz
c) I21.45 Iz
Where Ib = design current of circuit
In = nominal current of protective device
lz = current-carrying capacity of the cable
l2 = minimum operating current of protective device

Miniature circuit breakers and moulded case circuit breakers normally have tripping factors of, or below this 1.45 figure so that if either of these devices is used in compliance with condition a) above will mean that condition b) is also met, thus providing overload protection to the conductors concerned.

Protection Against Short Circuit
Short circuit is defined in the 16th Edition of the IEE Wiring Regulations as: "an overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions". IEE Wiring Regulation 434-03-02 states that: "provided an overload protective device complies with Regulation 433 and also provides short circuit protection the regulations are satisfied" without need for further proof. This is because if 433-03-02 is satisfied then the cable and the overload rating of the device are compatible. However, where this condition is not met or in some doubt for example where a protective device is provided for fault current protection only, as in an MCCB backing up a motor overload relay then IEE Wiring Regulation 434-03-03 must be satisfied "where a protective device is provided for fault protection only, the clearance time of the device, under short circuit conditions, shall not result in the limiting temperature of any conductors being exceeded."

Protection of Cables & Conductors Against Short Circuits
Regulation 434-03-03 of the IEE Wiring Regulations takes account of the time by applying what is known as the adiabatic equation 434-03-03 states:

"The time 't' in which a given short circuit current will raise the temperature of the conductors to the limiting temperature, can be calculated from the formula":-

Where t = duration in secs
s = cable cross section (mm²)
l = effective short circuit current (Amps)
k = a factor taking into account various criteria of the conductor

e.g. for a p.v.c. insulated copper conductor k = 115 (see table below) for a few of the k values quoted in the 16th Edition of the IEE Wiring Regulations.
Values of k for common materials, for calculation of the effects of short circuit current.

Therefore if the circuit breaker protecting the cable operates in less time than that required for the cable to reach its temperature limit the cable is protected (see example below, case A).

Assessment of protection under short circuit condition when based on the adiabatic equation is only accurate for faults of short duration e.g. less than 0.1 seconds as the equation assumes no heat loss from the cable.

IEE Wiring Regulation 434-03-03 also states that for a short circuit duration less than 0.1 seconds, where the asymmetry of the current is of less importance the value of k² s² for the cable should be greater than the energy let through (I²t) of the short circuit protective device (see below, case B).