Classes of Excess Current Protection with Examples
Table of Contents
Class of Excess Current Protection
According to the IEE regulations published in 1993, there are following two types of excess current protection.
(1). Close excess current protection
(2). Coarse excess current protection
All those cables, the insulation of which has been fabricated from any thermoplastic materials (e.g., PVC), if an excessive current above the allowed ratings, is transmitted through them even for a brief period of time, as a result, an intense temperature rise occurs in them. Remember that this temperature is quite more excessive than the allowed temperature produced as a result of continuous load on these cables.
As a result of this excessive temperature, ample possibilities of damage to the insulation of thermoplastic material cables, always exist. Thus, in order to protect cables from excessive current, the need for introducing the aforementioned regulation (i.e., close and coarse excess current protection) was felt. Through, these rules or regulations, not only the current ratings during the continuous usage of PVC insulated cables on allowed conductor temperatures, is determined, but information about the expected temperature resulting during the extreme current conditions can also be obtained.
The definitions of both types of excess current protection are as below;
Close Excess Current Protection
Such type of an excess current passes continuously for 4 hours through any fuse or other protective device for up to 1.5 times its deigned load, and during the meantime, that fuse or protective device operates (i.e., fuse link breaks, or the circuit breaker trips), is known as close excess current protection. In other words, an excess current that passes continuously through a fuse or other type of protective device e.g., a miniature circuit breaker, has its designed load increased by up to 1.5 times, for 4 hours. And, during the meanwhile, operates its protective device or its protection, which is called close excess current protection. Remember that in this type of protection, such HRC fuse links fuses, the fusing factor of which does not exceed 1.5 times, and such miniature circuit breakers, having an overload capacity, not over 1.5 times, are included. This type of protection is normally used in factories.
Coarse Excess Current Protection
Such an excess current, which can pass up to 1.5 times the designed load of a fuse or protective device for about four hours, and during the meantime that protective device does not operate, is known as a coarse (damaged or inferior) excess current protection. It should be remembered that re-wirable or, semi-enclosed fuses and fuse links are used in this type of protection. Further, this type of protection is mostly used in residential wiring.
Mutual Conversion of Close and Coarse Current Protection
(1). If the rating of a table designed for the coarse excess current protection is desired to be changed with respect to close excess current protection, the following formula is used for this purpose.
Current rating as close excess protection = 1.33 x current rating at coarse excess protection
In other words, the rating of close excess current can be determined by multiplying the current rating of coarse current protection by 1.33.
(2). If a table has been designed for close excess protection, and if we have to change the rating achieved from this table to coarse protection, then in such a situation, the following formula will be used.
Current rating as coarse excess protection = 0.75 x current rating at close excess protection
In other words, for conversion from close to coarse protection, the current rating obtained at close excess protection can be multiplied by 0.75 to obtain excess current protection.
Examples:
Example 1; A short length of three-core PVC non-armored cable having a copper conductor is required for a 3-phase A.C circuit having a continuous load current of 30A. it is to be installed on a cable tray unenclosed. The ambient temperature is 30°C. Find the size of cable?
(a). For coarse excess protection
(b). For close excess protection
Solution;
Load current = 30A, Ambient temperature = 30°C
(a). For Coarse Excess Protection
According to Pak. Cables & Tables under column 9 & 2
Nearest current rating = 33A
Cable size = 7 / .044”
(b). For Close Excess Protection
The table 14 is for coarse excess protection. For close excess protection, this table will be up rated by multiplying the rating factor (1.33)
Maximum current of cable 7/.044” for close excess protection = Rating factor x current rating for coarse excess protection
= 1.33 x 33 = 43.89 Amp.
This current rating is much greater in comparison with the load current of 30A. Now we try formal current rating in the table for suitable cable size.
Formal current rating = 27A, Cable size = 7/ .036”
Maximum current rating for close excess protection
= 1.33 x 27 = 35.9 Amp.
The cable 7 / .036” is suitable because its current carrying capacity is near to load current (i.e., 30A) Ans.
Example 2; A three-core armored PVC cable with aluminum conductors is required for a 3-phase A.C circuit having continuous load current of 200A. The cable is to be installed singly in defined conditions. The ambient temperature is 30°C. Find the size of cable?
(a). For close excess current protection
(b). For coarse excess current protection
Solution;
Load current = 200 A, Ambient temperature = 30°C
(a). For Close Excess Current Protection
A cable is armored with the aluminum conductor, table 20 is suitable, under columns 10 & 1.
Nearest current rating = 220A
Cable size = 0.2 inch2 (37 / .083”)
(b). For Coarse Excess Current Protection
Table 20 is for close excess protection. If coarse excess current protection is to be employed a 0.2 sq. inch (37/ .083”) cable will not be sufficiently large to carry 200 A, because it will have to be derated by multiplying the rating for close excess current protection with 0.75 9as mentioned under heading of rating factor of table 20).
Maximum current of 0.2sq. inch for coarse excess protection = 220 x 0.75 = 165 amp.
The cable is not suitable because its current rating is less than the load current of 200A.
Now select the next large size i.e., 0.3sq. inch (37/ 0.103”)
Maximum current rating for close excess protection = 290 A
Maximum current rating for coarse excess protection = 290 x 0.75 = 217.5 A
The cable 0.3sq. inch (37/ 0.103”) is suitable because its current carrying capacity is greater than the load current.
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