I. What is a fuse?
In electrical and electronics engineering, a fuse is an essential safety device that provides overcurrent protection of an electrical circuit. It is a means of protection (MOP) against electrical product hazards that helps remove power from a faulty system.
The composition of a fuse consists of a metal wire or strip that melts when the current flow exceeds the amount that the fuse is designed to withstand, consequently interrupting the current. The metal wire or strip is mounted between electrical terminals and often enclosed by a non-combustible housing. It is usually made of zinc, silver, copper, aluminium or alloys in order to provide stable and predictable characteristics.
Fuses have different dimensions depending on their market location. For instance, North American fuses should comply with UL 248-1 and/or CSA 22.2 no.248.1. According to these safety standards, fuses should come in two sizes: ¼ x 1 ¼ inch or 5 x 20 mm. However, in Europe, fuses must meet the requirements of EN 60127 (based on IEC 60127) and come in the 5 x 20 mm and 6.3 x 32 mm sizes. (Identify relevant EN & UL standards in minutes by using cloud-based product compliance tools.) In addition, each safety standard for fuses also dictates non-fusing and fusing time current limits.
II. Design hints for using fuses in EEE
When including fuses in the design of a product, electrical engineers should keep in mind the following tips:
- Fuses should be placed on the supply side of the mains circuits in the electrical products and should be preferably fitted in all supply conductors.
- A fuse shouldn’t be fitted in the protective earth conductor and neutral conductor of permanently installed products and multiphase electrical equipment.
- According to IEC 60127, fuses should withstand a current of 120% of product-rated current. However, North American standards have different requirements. They specify that the fuse must open with load conditions less than 110% of product-rated current.
- Each supply lead for Class I (grounded) products should include a fuse. The same applies to class II products with functional earth connections and other single-phase class II products.
- In some electrical products, the operation of the fuses should be detectable and indicatable.
- The type and rating of any operator-replaceable fuses should be specified adjacently to each fuse holder.
- The fuse voltage rating needs to be at least equal to the line voltage.
- Fuses compliant with IEC 60127 and used in primary circuits should have high breaking capacity (1 500 A) if the prospective short-circuits exceeds 35 A or ten times the fuse’s current rating.
- The fuse’s current rating should be based on the fault-mode current and be higher with 1.25 to 1.5 than the full load current.
- Any fuses compliant with IEC 60127 need to have the following markings: type of fuse, rated current and voltage. E.g. F1AH250V, meaning “fast blow-rated 1A/250V with high breaking capacity”.