The 16th Edition of the IEE Wiring Regulations (BS 7671) 533-01-06 requires that in an installation: "The characteristics and settings of devices for overcurrent protection shall be such that any intended discrimination in their operation is achieved".

Whether fuses or circuit breakers are utilised in a distribution system it is necessary to ensure that all the requirements of the 16th Edition of the IEE Wiring Regulations are complied with.

Discrimination, also called selectivity, is considered to be achieved when, under fault conditions the circuit breaker nearest the fault operates rather than any of the circuit breakers or fuses upstream of it (see below).

Concept Short circuit occurs at E - "A" remains fully closed. - "E" trips only, ensuring supply to B, C and D. The discrimination of circuit breakers can be based on either magnitude of fault (current discrimination) or the duration of the time during which the circuit breaker "sees" the fault current (time discrimination).

Current Discrimination in a distribution system requires a circuit breaker to have a lower continuous current rating and a lower instantaneous pick-up value than the next upstream circuit breaker. Current discrimination increases as the difference between continuous current ratings increases and as pick-up settings increase between the upstream and downstream breakers.

Time Discrimination in a distribution system requires the use, upstream, of circuit breakers with adjustable time delay settings. The upstream breakers must be capable of withstanding the thermal and electrodynamic effects of the full prospective fault current during the time delay.

Overload Discrimination
Time/Current discrimination at overload levels for products listed in chart 1.

Chart 1
Upstream	Downstream
BS 88 Fuse	Moulded Case or Miniature Circuit Breaker
Moulded Case or Miniature Circuit Breaker	Moulded Case or Miniature Circuit Breaker

At overload levels a comparison of the device time/current characteristic curves (see below) will show whether discrimination is achieved and if so the maximum value of fault current to which discrimination is achieved.

A 32SB3 Eaton MEM HRC fuse curve clears the 'knee' of a MCH116 Memshield 2 MCB curve and therefore will discriminate. The level to which discrimination is achieved is 250 amps derived by constructing a line from the end of the fuse curve (0.1 sec current) or as shown below where the fuse curve crosses the MCB curve. This shows that a 25A fuse and a 16A MCB downstream only discriminate up to 95A.

Short Circuit Discrimination:
Current discrimination at short circuit levels for products in the chart below.

Chart 2
Upstream	Downstream
BS 88 Fuse	Moulded Case or Miniature Circuit Breaker

Where high prospective fault levels exist at the circuit breaker distribution point then discrimination at short circuit levels should be considered. This requires comparison of the devices total let through energy and pre-arcing energy for the prospective fault level concerned.

Discrimination will be obtained at all fault levels for the circuit breaker when its total let through energy (I²t) is less than the pre-arcing energy (I²t) of the fuse nearer the supply.

The information for Eaton MEMs BS 88 HRC fuse range can be extracted from curves and is presented in tabular form here. This can be compared with Memshield 2 miniature circuit breakers and moulded case circuit breaker total let through energy curves an example being shown below.

The total let through energy of a 32A Memshield 2 miniature circuit breaker experiencing a fault of 5kA will be 22000A²s (see above graph). Relating this value to the pre-arcing value of the upstream fuse (see chart 2) it can be seen that the lowest rated fuse providing discrimination is the 125SF6, as its pre-arcing energy is greater than the total let through energy of a 32A Memshield 2 MCB at 5kA ie.

Fuse pre-arcing		MCB Total let through
Upstream		Downstream
29743A²s	>	22000A²s
Fuse		Circuit Breaker

Full discrimination is achieved at 5kA.

This has been calculated for every combination of Memshield 2 circuit breakers and Eaton MEM BS 88 fuselinks - click here.

Short Circuit Discrimination:
Current discrimination at short circuit levels for products in the chart below.

Chart 3
Upstream	Downstream
Category A MCCB	Category A MCCB
Category A MCCB	MCB

Category A moulded case circuit breakers are defined un BS EN 60947-2 (IEC 60947-2), summarised as follows:-

Category "A" applies to circuit breakers not specifically intended for selectivity (discrimination) under short circuit conditions.

Discrimination is possible but not on a time basis. These are current limiting type moulded case circuit breakers and as such it is not possible to assess short circuit discrimination by overlapping time current curves. Discrimination in the overload portion of the time/current characteristic can be shown by overlapping the time current curves but to determine short circuit discrimination a different technique must be applied.

Discrimination between two circuit breakers both of category A current limiting type cannot be determined by comparing the individual I²t figures of the circuit breakers. This is not possible because unlike fuses, circuit breakers have no "fixed" pre-arcing energy. The nearest equivalent is the delatching energy; the point at which the tripping mechanism starts to open and is past its "point of no return".

The diagram below shows a typical fault current trace for a Memshield 2 current limiting MCB or MCCB. It can be seen that the delatch time (O-t0) and hence the energy let through for that period, is considerably less than that for the period of time (O-t2) that it takes to completely break the fault. Utilising the pre-arc energy - delatching energy analogy it is apparent that comparison between two current limiting Category A circuit breakers would represent less favourable results as the delatching I²t energy would rarely be greater than the total let through energy of the downstream device.

Utilising the peak let-through current curve (in the diagram above) it is possible to extrapolate the level to which a current limiting circuit breaker will limit a prospective fault.

Examination of peak let-through current curves show a G Frame Memshield 2 MCCB will limit a 11kA fault to 11kA peak = 7.8kA RMS.

If the RMS equivalent value of the peak cut off current of the downstream circuit breaker is lower than the magnetic setting of the upstream circuit breaker then discrimination is assured (see example below).

From the time current curve the discrimination level appears to be 8kA.

Examination of peak let-through curves shows that 63A 'G' Frame Memshield 2 moulded case circuit breakers will limit a 11kA prospective fault to 11kA peak =7.8kA RMS.

Therefore at 11kA the equivalent current let-through of the downstream breaker does not exceed the magnetic takeover level of the upstream breaker.

Peak let-through of downstream 63A 'G' frame = 7.8kA RMS

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Magnetic takeover level of upstream 800A 'L' frame = 8.0kA RMS.

This means we have a discriminating system to 11kA.

Short Circuit Discrimination:
Time/Current discrimination at short circuit levels for products in the chart below.

Chart 4
Upstream	Downstream
Category B MCCB	Category A MCCB
Category B MCCB	Category B MCCB
BS 88 Fuse	Category B MCCB
Category B MCCB	MCB
Category B MCCB	BS 88 Fuse

Category B moulded case circuit breakers are defined in BS EN 60947-2 (IEC 60947-2), summarised as follows:-

Category "B" applies to circuit breakers specifically intended for selectivity under short circuit conditions with respect to other short-circuit protective devices in series on the load side.

These circuit breakers are equipped with an intentional short time delay. This ensures that the upstream circuit breaker remains closed long enough under short circuit conditions to allow the downstream circuit protective device to clear the fault (see diagram below).

In contrast with the current limiting category A type circuit breakers this type of circuit breaker is designed to withstand the rated short time withstand current (Icw) for the time duration dependent on the maximum time delay setting made on the circuit breaker.

Time Discrimination:
"The total clearing time of the downstream breaker must be less than the time delay setting of the upstream breaker".

The upstream circuit breaker must have a sufficient withstand capability for the thermal and electrodynamic effects of the full prospective short circuit.

To determine discrimination utilising an upstream category B moulded case circuit breaker is relatively simple, it is only necessary to compare time/current characteristics with those of the downstream device and ensure that no overlap occurs.

Back-up Protection:
Back-up (Cascading) is recognised and permitted by the 16th Edition of the IEE Wiring Regulations (BS 7671) 434-03-01.

"A lower breaking capacity is permitted if another protective device having the necessary breaking capacity is installed on the supply side. In this situation, the characteristics of the device shall be co-ordinated such that the energy let through of these two devices does not exceed that which can be withstood without damage by the device on the load side and the conductors protected by these devices".

Back-up can be obtained with moulded case circuit breakers by the utilisation of the current limiting capacity of the upstream circuit breaker to permit the use of the lower breaking capacity and therefore lower cost downstream circuit breaker provided that the breaking capacity of the upstream circuit breaker is greater than or equal to the prospective short circuit current at its point of installation (see below).

In response to a short circuit fault the operation of the upstream circuit breaker creates an impedance which in conjunction with the impedance of the downstream device enables the downstream device to handle the short circuit potentially possible at this point of application.

Example:
By installing a Memshield 2 'F' frame MCCB (25kA breaking capacity) at the upstream end of the installation and with an Isc of 20k on the busbars, it is possible to install Memshield 2 Type B, C or D, characteristic 1 - 63A MCBs (10kA breaking capacity) on the outgoing lines.