Protective Devices Coordination and Discrimination
Discrimination between two protective devices in series is, in most cases, considered to be achieved when overcurrent conditions occurring on the load side of the downstream device results in the operation of that device leaving upstream devices still providing supplies to the remaining healthy circuits. Discrimination is sometimes also referred to as ‘selectivity’ which is defined as ‘the choosing of one (not all) and the exercising of judgement in making the choice’.
In most installations, discrimination is needed between series-connected or cascaded protective devices to prevent danger or to minimise inconvenience.
Earth Fault Discrimination
Fig 1 below illustrates a small installation with four final circuits. Each final circuit has its own protective device (labelled B to E in the figure). In addition, a protective device A is installed in the supply to the four final circuits.
In this example, the installation designer requires discrimination to be achieved for earth faults. Consider an earth fault occurring in circuit No 4 on the load side of protective device E. The fault current will be ‘seen’ by devices E and A. Discrimination is achieved when, under earth fault conditions, protective device E operates and protective device A does not. The upstream device A still provides supplies to the three remaining healthy circuits (circuits 1, 2 and 3).
The installation designer will select suitable devices such that the intended discrimination is achieved.
Discrimination may be required between different types of cascaded protective device, such as fuses, circuit-breakers and residual current devices (RCDs). For example, a circuit-breaker may be required to discriminate with a fuse. Even where protective devices are the same type of device, such as circuit-breakers, they may be manufactured to different Standards, or be manufactured to the same Standard but be a different type within that Standard (such as Type B or Type C to BS EN 60898). The installation designer must ensure that any discrimination required between series-connected devices is achieved.
The definition of overcurrent discrimination is ‘Co-ordination of the operating characteristics of two or more overcurrent protective devices such that, on the incidence of overcurrents within stated limits, the device intended to operate within these limits does so, while the other(s) does (do) not.’
The implication of this definition is that, for a given value of overcurrent, it may not always be the device nearest the source of the overcurrent that is required to operate.
For example, in Fig 2, an upstream BS 88 fuse (with a rated short-circuit breaking capacity of 80 kA) is providing back-up protection to a circuit-breaker (rated short-circuit breaking capacity of 5 kA). A fault occurs on the load side of the circuit-breaker. The prospective fault current is, in this example, taken to be 10 kA. A fault current of this value would be expected to damage the circuit-breaker and, therefore, under such conditions, the installation designer will require the upstream device, the BS 88 fuse, to operate to disconnect the circuit (Regulation 434.5.1 refers).
Under certain conditions, the upstream device is required to operate:
Requirements placed by BS 7671
Every installation is to be divided into circuits to avoid danger and to minimize inconvenience in the event of a fault. Where an installation includes cascaded protective devices, discrimination will, in most cases, be necessary either to avoid danger or to minimize inconvenience.
To avoid danger
As stated above, discrimination between protective devices is required where necessary to avoid danger. BS 7671 places further specific requirements applicable to overcurrent protective devices and residual current devices.
Overcurrent protective devices. The characteristics and setting of devices for overcurrent protection are to be such that, for fault currents within stated limits, any intended discrimination is achieved. Overcurrent protective devices include fuses and circuit-breakers.
Residual current devices. Discrimination may be required where two or more RCDs are in series. The intended discrimination is achieved by selecting devices with appropriate characteristics. For example, the installation designer may consider the selection of a time delayed device for the upstream device.
To minimize inconvenience
Discrimination may also be required to minimize inconvenience in the event of an overcurrent such as that due to a fault, for example, by restricting the consequences of an overcurrent condition to the circuit concerned.
Fig 3 illustrates a small domestic installation with four final circuits. Discrimination should normally be assured such that a fault on the cooker final circuit causes disconnection at protective device E and not at protective device A. Disconnection at protective device A would cause loss of supply to all circuits including the lighting circuit, and this would almost certainly result in inconvenience, particularly if the cooker fault occurred at night.
Inconvenience must be minimized
Designing for the correct discrimination
In most installations the required discrimination is normally achieved when the minor or downstream device is the one that operates under overcurrent conditions. There are instances where the way in which devices discriminate may be required to change according to the value of the overcurrent detected. For example the downstream device may be designed to operate under overload conditions and the upstream device to operate under fault conditions.
In order to carry out an analysis of an electrical distribution system with respect to discrimination of protective devices detecting overcurrent or residual current, it is necessary to have available details of the operating characteristics of all the protective devices concerned, together with all relevant values of current. In most cases it is necessary to know the prospective fault current at the point of installation of each device. This however is not necessarily the only value of current required. For example where the way in which devices are required to discriminate changes according to the value of the overcurrent detected, it is necessary to know the value of current at which the change in discrimination is to occur. This may be, for example, the rated short-circuit breaking capacity of a circuit-breaker or the rated breaking capacity of a contactor in a motor starter.
Having obtained the appropriate data, it is necessary to determine whether the correct discrimination will be achieved. The installation designer is often required to address the discrimination between:
- Circuit-breaker and HBC fuse
- HBC fuses
Discrimination between circuit-breakers connected in series is achieved when the downstream circuit-breaker completes its breaking operation in time to prevent the upstream circuit-breaker from starting its operation, i.e. the point at which the upstream circuit-breaker’s opening command becomes irreversible.
Discrimination between a circuit-breaker and an HBC fuse
Two configurations require consideration. In the first configuration the upstream protective device is a circuit-breaker, the downstream device an HBC fuse. In the second configuration, the positions of the two protective devices are reversed. The second configuration is illustrated in Fig 2. Discrimination between a circuit-breaker and an HBC fuse connected in series is achieved when the circuit-breaker completes its breaking operation before the end of the pre-arcing time of the fuse. Discrimination between an HBC fuse and a circuit-breaker connected in series is achieved when the fuse has extinguished the arc before the circuit-breakers opening command becomes irreversible.
Discrimination between HBC fuses
Discrimination between fuses connected in series is effected when the total operating of the minor (downstream) fuse is less than the pre-arcing of the major (upstream) fuse.
Discrimination between RCDs
In the event of an earth fault, the earth fault loop impedance is generally such that the earth fault current is in excess of the current required to operate any Residual Current Device (RCD). It is therefore not practical to achieve discrimination on the basis of magnitude of current. Discrimination between RCDs can really only be accomplished by delaying the time before operation of the upstream RCD, such that the downstream RCD has time to disconnect the fault, before the time delay of the upstream RCD has expired. London Electrical Inspection & Testing can design and specify your protection devices so as to ensure discrimination and full compliance with the Wiring Regulations.
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