Where discrimination occurs for any level of fault current up to the system prospective, then discrimination is said to be total. A discrimination limit defines the size of fault current level up to which discrimination is maintained.
In many cases, protective device selection aimed at total discrimination may result in greater expense, both in cable sizes, and types and ratings of circuit protective devices. For certain applications the extra cost will be justifiable, but often a more economical choice can be made without any significant loss of discrimination under normal operating conditions.
Areas where total discrimination is essential include:
An essential part of a network design is a discrimination or grading study depicting the characteristics of the relevant protective devices. Using such a study, levels of discrimination can be visually assessed. The designer can then decide whether the level of discrimination achieved with a given selection of protective devices and settings is appropriate, or whether changes need to be made.
Software is available which is designed to reduce the time spent on the preparation of discrimination studies.
Different types of discrimination
Generally discrimination between two protective devices can be determined by comparing their tripping characteristics on a log/log time/current graph. If there are no overlaps between characteristics, discrimination can be assumed to be good for all fault current levels. Any overlap highlights a region of fault current in which discrimination may fail.
For non-current limiting protective devices, this is the only check necessary to determine discrimination.
For current limiting protective device types the graphical check still holds good for nearly all fault conditions but a separate check is necessary to verify discrimination under short circuit conditions.
Current limiting protective devices
Current limiting devices include fuses, miniature circuit breakers (mcbs) and some moulded case circuit breakers (mccbs).
Current limiting protective devices are designed to operate very rapidly (<10 ms) under fault current conditions. Such action severely limits the energy let through the device during a fault, minimising damage to the system and also reducing the peak value of fault current.
Current limiting protective devices have two main advantages over their equivalent non-current limiting protective devices – they offer greater protection to the network under fault conditions and they are physically much smaller. However, because they operate more rapidly, discrimination can be more difficult to achieve under short-circuit conditions.
Discrimination between current limiting devices under short-circuit conditions can only be assessed by consideration of the level of energy (A²s) flowing through each device. This has to be considered in addition to the display of the time/current characteristics on a graph. Therefore, clearance between two current limiting devices on a graph does not necessarily demonstrate total discrimination.
LV current limiting circuit breakers manufactured to IEC 947-2 are classified as Category A, ie they are:
Non-current limiting protective devices
LV air circuit breakers (acbs) are generally non-current limiting types and are designed to remain closed for a settable delay, regardless of the level of fault current, until tripped by an internal or external protection relay. Such devices, fitted with short-circuit protection time delays, allow discrimination to be achieved across a number of levels of distribution. These circuit breakers are classified in IEC 947-2 as Category B, ie they are:
Because of the ability of lv acbs to provide time delayed operation under short-circuit fault conditions, they can be used to give total discrimination without difficulty. Time delays can be set (typically between 100 ms and 500 ms) to allow discrimination on a time basis to be achieved over several levels of distribution.
Because of the requirement to sustain high prospective fault currents for some period before disconnecting, acbs are larger and more expensive for a similar load current capacity, compared to mccbs or mcbs.
Discrimination assessment with current limiting protective devices
Clearly, where current limiting protective devices are used, a further check to verify discrimination under short-circuit conditions is necessary, in addition to a graph.
This additional check is only necessary when evaluating discrimination under short-circuit conditions between two current limiting devices. Where two circuit protective devices are considered, if the upstream protective device is non-current limiting and, providing there is no overlap of the two characteristics on the graph, discrimination will be ok.
Various methods of assessment of discrimination between current limiting circuit protective devices are available depending on the types of products under consideration. Fuse manufacturers may give a simple ratio to be applied to the ratings of upstream and downstream fuses, for example 1·6:1. Using this ratio, the smallest upstream fuse that could be used with a 100 A fuse downstream would be 100 x 1·6 = 160 A.
More generally, discrimination between fuses can be determined by considering the level of energy (A²s) flowing under fault conditions. To ensure discrimination, the energy let through by the downstream fuse (total I²t) must be less than the amount necessary to melt the element of the upstream fuse (pre-arcing I²t). The I²t values can be obtained from tables provided by the fuse manufacturer.
Guidance on protective device settings
In the initial design stages of a network it may be easier to select the circuit-breakers than to arrive at the optimum protection settings that need to be made to ensure that both circuit protection and discrimination are achieved.
The following points may help when making adjustments to protection settings:
1. For final loads, short circuit settings should be high enough to avoid the possibility of tripping for the highest foreseeable level of non-fault current, eg motor starting current, transformer magnetising current etc;
2. Good practice dictates that all settings should be made as low as possible. Making high protection settings may increase the damage sustained by the network under fault conditions;
3. When making short-circuit settings, remember that a setting higher than the prospective fault current at the point at which the protective device is installed, will not operate;
4. Finally, ensure clearance between each successive level of protection, so that discrimination is maintained. If this is not possible at all levels a solution might be found by substituting one or more critical circuit-breakers with types having a wider range of settings.
You may encounter discrimination problems that are inherent in a poorly designed network. For example, if a distribution board protected by a 200 A circuit breaker has an outgoing way with a rated current of 160 A, it may only be possible to achieve total discrimination using two circuit breakers with electronic protection.
Source
Electrical and Mechanical Contractor
Postscript
Roy Hughes is senior support engineer with Amtech Computer Systems.
