What is the difference between discrimination and backup protection? This is a question that has troubled some.
The fact that, ‘selectivity’ is now the preferred word for discrimination and that back-up protection is also called, ‘cascading’ doesn’t help.
Add related terms like, ‘pre-arcing energy’ and ‘energy let-through’ and it’s not difficult to understand the potential for confusion.
Co-ordination of protective devices
Series-connected protective devices, as shown in Fig. 1, are normally co-ordinated to prevent danger and to ensure proper functioning of the installation. (The exception is where disconnection of the supply is not permitted, as in the case of life-support systems.) Regulation 536.1 of BS 7671 indicates that the word, ‘coordination’ generally means consideration of selectivity and /or any necessary back-up protection. Clearly then, selectivity and back-up protection are not the same and we will now consider each one.
Discrimination (Selectivity), is defined in Part 2 Definitions as, ‘Ability of a protective device to operate in preference to another protective device in series’.
The purpose of selectivity is to isolate a part of an electrical installation with a fault condition from the rest of the installation such that only the protective device located immediately on the supply side of the fault operates to disconnect the supply.
The traditional means of achieving selectivity are by current or by time.
Current selectivity utilises overload or fault current while time selectivity uses devices with different trip time settings.
Current selectivity is the more common method and the one which we will deal with here.
There has frequently been discussion within the industry about which devices will discriminate with which other devices and at which level of fault current.
Regulation 536.2 requires that the manufacturer’s instructions be taken into account in the design of selectivity between overcurrent protective devices.
For selectivity between fuses, the rule of thumb is that an upstream fuse will discriminate with a downstream fuse of half its rating.
For selectivity between circuit- breakers, the 2 to 1 ratio is generally valid for the thermal trip section of the curve.
Selectivity is a basic requirement for a reliable and safe installation.
The pre-arcing energy and the energy let-though
It will be appreciated that protective devices do not operate instantaneously and, after a device starts to open, some fault current will continue to flow through it for a certain time. To explain selectivity further, we must introduce two terms – the pre-arcing energy and the energy let-through.
Both terms are important characteristics in the design of selectivity (and backup protection systems) and both can be seen in Fig. 2.
At point A (time t1), the contacts start to open and arcing begins
At point B (time t 2), disconnection begins
At point C (time t 3), the fault has cleared
Area 0B t 2 portrays the pre-arcing energy
Area 0B t 3 represents the total energy let-through
The pre-arcing energy I2t is the energy required to make a fuse element start to melt or contacts begin to open. (The term, ‘melting’ only applies to fuses but all overcurrent protective devices develop an arc when their circuit contacts open. Thus, the term ‘pre-arcing’ applies generally to overcurrent protective devices.) The total energy let-through I 2t is the total energy which is let through by the device until the arc is quenched. (The unit of I2t is amps squared per second.)
Selectivity is achieved when the pre-arcing I 2t of the upstream device is less than the total energy let-through I 2t of the downstream device.
In plain English – for fuse to fuse discrimination – this means that the smaller fuse must complete its operation before the larger fuse reaches its melting point.
Selectivity between fuses is shown in Fig. 3.
In terms of cable protection, the I²t must be less than or equal to K²S² which is the ‘energy withstand’ of the cable. (See Regulation 434.5.2 for an explanation of these terms.)
BS 7671 requires that the rated breaking capacity of each protective device must be not less than the maximum prospective short-circuit current or earth fault current at the point at which the device is installed – unless backup protection is provided.
The basic principle of back-up protection is contained in two regulations:
- Regulation 434.5.1, which deals with the characteristics of fault protective devices, and,
- Regulation 536.1, which concerns the coordination of protective devices
The term, ‘energy let-through’ is mentioned in both of these regulations in the context of backup protection.
Back-up protection is provided where the characteristics of the devices are suitably co-ordinated such that the energy let-through (I 2t) of the device on the upstream side (ie. the supply side) does not exceed that which can be withstood, without damage, by the device(s) on the downstream side (that is, the load side).
Back-up protection permits the installation of downstream devices (and cables and enclosures) which have a short-circuit breaking capacity lower than the prospective short-circuit current where they are installed, giving substantial savings.
Perhaps back-up protection is best illustrated by the cartridge fuses (BS 1361 or BS 88-3) in domestic cut-outs.
Protective devices in a consumer unit, such as circuit-breakers and RCBOs with a rated short-circuit capacity of 6kA or 10kA and rewireable fuses to BS 3036 with a rated short-circuit capacity of 1 – 4 kA are ‘backed up’ by the cartridge fuse in the cut-out.
Where backup protection is provided, it must be in accordance with the manufacturer’s information.
The design of selectivity and backup protection are greatly assisted by studying the graphs in Appendix 3 of BS 7671 and /or seeking guidance from the relevant manufacturer.
While it is hoped that this article has proved helpful, it should be considered only as an introduction to these topics.