One of the long standing advantages of fuse protection is that fuse selection is relatively simple and effective.
The following notes should cover the majority of applications. Reference should also be made to the appropriate Wiring Installation rules, such as the IEE Wiring Regulations for Electrical Installation of buildings (BS7671) which aligns with IEC60364.
The current rating of the fuse-link should not be less than the full load current of the circuit. The circuit should be so designed that small overloads of long duration will not be of frequent occurrence.
Protection Against Electrical Shock
The rules for pro tection against indirect contact are given in Chapter 41 of the wiring regulations. For a TN System a disconnecting time not exceeding 5 seconds is permitted for a distribution circuit. The maximum values of each fault loop impedance (Zs) of 240V for gG fuse-links to BS88: Parts 2 and 6 are shown below.
|Rating (A)||Zs (Ohms)||Ratings (A)||Zs (Ohms)|
Above an ambient of 40ºC a general de-rating of 0.5% of the
fuse-link rated current per excess degree ce ntigrade is
The standardized values of Breaking Capacity for fuse-links to BS88 are 80kA for voltages of 415V a.c., and above, and 40kA for d.c., applications. The 240V a.c., designs have a breaking capacity of 16kA minimum.
Harmonised Voltage Ratings
The voltage ratings of all Lawson fuses are compatible with the proposed harmonised voltage ratings of 230V a.c. , 400V a.c., and 690V a.c.
All fuse-links to BS88 have a discrimination ratio of 1.6: I. For example, an upstream fuse-link rated at 160A will discriminate with a downstream fuse-link rated at 100A.
Current and Energy Limitation
The Lawson range of fuse-links have pre-arcing I2t values towards the lower limits of BS88 ensuring excellent current and energy limitation. They also have power losses at rated current significantly lower than the standardized limits which assists in the appropriate interchangeability with other makes of fuse-links.
Cable Ratings and Protection
Lawson fuse-links with gG characteristics protect associated cables against both overload and short circuit currents, provided that the current rating of the fuse-link In is equal to or less than the current carrying capacity of the cable Iz.
There is an increasing move away from 70ºC pvc insulation to materials, which are more environmentally friendly for example 90ºC XLPE. The ratings of fusegear, switches, accessories etc. are generally based upon the equipment being connected to conductors intended to be operated at a temperature not exceeding 70ºC in normal service.
In view of the above it is recommended that the practice of designs based upon conductor temperatures of 70ºC be regarded as the norm. In accordance with the Wiring Regulations, the equipment manufacturer should be consulted to ascertain the reduction of nominal current rating of the equipment if conductor temperatures exceeding 70ºC are used.
|Cable Size||Max. Fuse Rating|
|(mm2)||K=115 A||K=143 A|
In motor circ u i t s, the motor starter provides the overload protection and the fuse-link provides the short circuit protection. The maximum size of fuse-link that can be used depends upon the type of cable used and is determined in accordance with the Wiring Regulations using the appropriate K factor.The table gives the maximum sizes of fuse-links that are recommended for two popular cables with copper conductors, 70ºC pvc (K = 115) and 90ºC thermo-setting (K =143).
When fuse-links are used on the primary side of transformers the normal current rating of the fuse-link should be at least twice the nominal transformer primary current.
The normal current rating of the fuse-link should be at least twice the normal full load current of the maximum number of lights to be switched simultaneously.
In capacitor circuits, eg. power factor correction, the fuse-link should be chosen with a current rating greater than 1.5 times the rated capacitor current. This takes account of the high transient inrush current, circuit harmonics and capacitor tolerances.
Type 2 Co-ordination
Motor starter manufacturers recommend the fuse-link rating that can be used in conjunction with motor starters. These recommendations usually do not state any specific fuse-link manufacturers type number and they often refer to gG fuse-links in accordance with IEC 60269 or the equivalent national standards e. g. BS88: part 2. This gives a simple and effective means of co-ordination since a fuse-link selected in accordance with manufacturers’ recommendations to withstand inrush currents will normally give adequate short circuit protection to the motor starter.