For example, in a domestic installation, a fault in a ring final circuit should result in the protective device for that circuit operating while lighting and other circuits remain energised. If the fault resulted in operation of the supply authority's service fuse, loss of power for lighting and other circuits could present a safety risk and would result in unnecessary cost and inconvenience for the householder.
Discrimination may apply to:
- Overcurrent protective devices (circuit-breakers and/or fuses).
- Residual current devices.
Where residual current devices are used in cascade, for example as the main switch in a domestic consumer unit and for protection of individual final circuits downstream, special care is needed to ensure discrimination.
BS 7671, The IEE Wiring Regulations, states quite simply, for overcurrent protection or residual current protection devices, that any intended discrimination in their operation must be achieved. But how is that need determined?
Regulation 531-02-09: Where, for compliance with the requirements of the Regulations for protection against indirect contact or otherwise to prevent danger, two or more residual current devices are in series, and where discrimination in their operation is necessary to prevent danger, the characteristics of the devices shall be such that the intended discrimination is achieved.
Regulation 533-01-06: Where necessary, to prevent danger, the characteristics and setting of a device for overcurrent protection shall be such that any intended discrimination in its operation is achieved.
Forms of discrimination:
There are three principal aspects to discrimination:
- Overload discrimination relates to the magnitude of the fault current - for this the upstream device must always have a higher continuous current rating and a higher instantaneous pick-up value than the next device downstream.
- Short-circuit discrimination must be considered in any situation where high prospective fault levels exist. This occurs where the earth fault loop impedance is low, for example if the installation is close to the local transformer substation. A short-circuit at, or close to, the protective device will involve exceptionally high energy levels (bearing in mind that energy, I2t, is related to the square of the current).
- Time discrimination relates to the time during which the circuit-breaker 'sees' the fault current. This requires the use of adjustable time delay settings in upstream devices. In addition the upstream device must be able to withstand the thermal and electrodynamic effects of the full prospective fault current during the delay period.
Fuse and circuit-breaker:
Fuses are frequently used in conjunction with circuit-breakers because they have a higher short-circuit breaking capacity. However, the two devices present quite different time-current characteristics.
Manufacturers produce time/current curves for their fuses and circuit-breakers. Curves are also produced in Appendix 3 to BS7671, where they are based on the slowest operating times for compliance with the relevant standards.
By comparing the curves for the two devices, it is possible to determine whether full overload discrimination is achieved or, if not, at what levels of overload current it is achieved.
This is illustrated in Figs 1 and 2. Fig. 1 compares the time/current characteristics of a 16A miniature circuit-breaker with those of a 32A fuse; the fuse curve clears the "knee" of the circuit-breaker curve and so discrimination is achieved with the fuse upstream of the MCB. However, in Fig . 2, which uses a 25A fuse instead of the 32A fuse, discrimination is only achieved up to 95A. Beyond this the fuse would blow before the circuit-breaker opened.
Short circuit discrimination involves comparison of the total let-through energy and pre-arcing energy for the prospective fault level concerned. Discrimination is achieved if the total let-through energy of the circuit-breaker is less than the pre-arcing energy of the fuse, at all values of fault level.
Fuse manufacturers produce tables listing the pre-arcing energy of their devices while circuit-breaker manufacturers produce curves presenting the total let-through energy at different values of prospective fault current.
Fig. 3 shows the maximum total let-through energy for a range of miniature circuit-breakers. From this it will be seen that the total let-through energy of a 32A MCB experiencing a fault of 5kA will be 22,000A2s. The same manufacturer's fuse data shows that for BS88 HRC fuse-links, the pre-arcing energy levels are:
- 100A fuse - 20655 A2s
- 125A fuse - 29743 A2s
- 160A fuse - 46474 A2s
- 200A fuse -118973 A2s.
From this it will be seen that a fuse rating of 125A or greater will achieve short-circuit discrimination with the downstream 32A MCB.
When it comes to moulded case circuit-breakers the situation will depend on the type of device.
- Category A - MCCBs are energy-limiting devices which are not specifically intended for discrimination under short-circuit conditions. Discrimination is, however, possible by examining the peak let-through current curves; if the RMS equivalent of the peak let-through current of the downstream circuit-breaker is less than the magnetic takeover level of the upstream device, discrimination will be achieved.
- Category B - MCCBs are specifically intended for discrimination under short-circuit conditions with respect to other short-circuit protective devices (Category A or B MCCBs, MCBs or fuses) on the load side. These devices have an intentional time delay which ensures that they remain closed long enough, under short-circuit conditions, to allow the downstream device to clear the fault. To achieve this, the Category B MCCB is designed to withstand the rated short time withstand current (Icw) for its maximum delay time.
Time discrimination with an upstream Category B MCCB is simply a matter of comparing the time/current characteristics of the breaker with those of the downstream device and ensuring that no overlap occurs.
To simplify the whole issue of determining whether discrimination is achieved with different combinations of fuse, MCCB and/or MCB, MEM Circuit Protection and Control publishes tables listing the prospective fault levels to which discrimination is achieved. The user simply looks up the device and rating for the upstream device on one axis and the type and rating for the downstream device on the other axis. If discrimination is possible, the maximum prospective fault level up to which it is possible, is indicated at the intersection of the two columns.
Residual current devices:
When two or more residual current devices are used in succession, it is necessary to use a time delayed upstream device to achieve discrimination. This is because the operating characteristics of the RCD are quite different from those of a fuse or circuit-breaker. If an RCD sees a fault in excess of its rated tripping current, it will trip irrespective of the level of the fault. So a 100mA device and a 30mA device in cascade will both trip if there is a fault of, say, 500mA.
Time delayed 100mA and 300mA RCDs are available in addition to standard 'instantaneous' devices. Typically, standard 10mA, 30mA, 100mA or 300mA devices will trip within 100ms while time-delayed devices will trip within 300ms at earth leakage currents equal to their rated sensitivity.
These comments apply equally to the residual current protection offered by combined MCB/RCD units (RCBOs).
The electrical contractor should not be put off by the apparent complexity of pages of time/current characteristics and tables of pre-arcing and total let-through energy for fuses and circuit-breakers. Overload discrimination and time discrimination can be determined relatively easily. Even short-circuit discrimination has been simplified by the availability of tables indicating the level of discrimination achieved for each combination of devices.
The pictures show the interior of an Eaton Series G MCCB, and the same device with front-fitted components.