HAVE YOU EVER BEEN ASKED … CAN REWIREABLE FUSES STILL BE USED? ewireable (semi-enclosed) fuses to BS 3036 are commonly found in consumer units, distribution boards and other switchgear in older domestic and non domestic installations – particularly installations more than 20 years old.
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Where an installation having rewireable fuses is periodically inspected, altered or extended, the inspector or designer (as applicable) will have to consider whether BS 7671 still allows such fuses to be used, or whether they should be replaced with circuit breakers or cartridge fuses.
Drawing courtesy of British Standards Institution
Can rewireable fuses still be used? In a word, yes. BS 7671 permits the use of rewireable fuses complying with BS 3036 (although Regulation 533.1.1.3 prefers fuses to be of the cartridge type). Therefore, rewireable fuses can still be used, for existing and new installations. Related considerations A number of considerations relating to the use of rewireable fuses are listed below. Maintainability With rewireable fuses there is a risk of the fuse wire being replaced, inadvertently or deliberately, with one having a higher fusing current than intended for the circuit, resulting in reduced effectiveness of the protection given by the fuse. This risk is usually acceptably low, even in domestic premises. Many millions of rewireable fuses have provided satisfactory service in the UK over many years.
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Nevertheless, the inspector/designer should assess the degree of risk, to help him/her judge if there are any special circumstances that would make rewireable fuses unacceptable in the particular installation. Account should be taken of an assessment of the frequency and quality of maintenance that the installation can reasonably be expected to receive, which is called for by Regulation 341.1. Fusing factor BS 3036 allows a rewireable fuse to have a fusing factor (the ratio of the minimum fusing current to the rated current) of up to 2.0. This value is higher than the typical value of 1.25 to 1.6 for other types of device commonly used for overload protection (such as fuses to BS 1361 or BS 88, and circuit breakers to BS EN 60898 or BS EN 60947 2), depending on the type. To compensate for this high fusing factor where a rewireable fuse is used for overload protection, Regulation 433.1.3 requires the current-carrying capacity of the circuit cable to be not less than In divided by 0.725 (where In is the rated current of the fuse). This can mean that a larger cable size is needed than if another type of overload protective device of the same rated current had been used. However, this is not the case for a 30 A or 32 A ring final circuit, where Regulation 433.1.5 allows a minimum size of 2.5 mm2 (copper) for the line and neutral conductors of the circuit, even if the protective device is a BS 3036 rewireable fuse (rated at 30 A). The requirement for the cable currentcarrying capacity to be not less than In divided by 0.725 does not apply for a circuit where overload protection may be omitted (see Regulation Group 433.3). An example is a circuit supplying only an electric heater, as the circuit would be unlikely to carry overload current (Regulation 433.3.1(ii) refers). However, the cable must still have a current carrying capacity of not less than the load current (I∆), and protection against fault current must still be provided for the cable (Section 434 of BS 7671 refers).
typically 6 kA or more for modern circuitbreakers to BS EN 60898, or over 16.5 kA for cartridge fuses to BS 1361 or BS 88, depending on type.
Table 1 – Rated breaking capacity of BS 3036 rewireable fuses Category of duty
Breaking capacity
S1A
1 kA
(at 0.8 to 0.9 power factor, lagging)
S2A
2 kA
(at 0.7 to 0.8 power factor, lagging)
S4A
4 kA
(at 0.7 to 0.8 power factor, lagging)
Notes: 1. BS 3036 fuses in consumer units typically have a rated short-circuit breaking capacity of 2 kA. 2. BS 3036 fuses are also tested in circuits with a power factor of 0.4 to 0.5 lagging at a current of between 5 and 7 times the minimum fusing current No less than any other fault current protective device type, rewireable fuses must be chosen to meet the breaking capacity requirements of Regulation 434.5.1. That is to say, the rated short circuit breaking capacity must be not less than the maximum prospective fault current at the point where the fuse is installed, except where back up protection meeting specified requirements is provided by another device. It can be difficult to meet the requirements of Regulation 434.5.1 using BS 3036 fuses, due to their low breaking capacity, shown in Table 1. However, where incorporated in a consumer unit, BS 3036 rewireable fuses are considered adequate for prospective fault currents of up to 16 kA, provided the consumer unit: to BS EN 60439-3 including • conforms Annex ZA, and fed by a single-phase supply of nominal • isvoltage 230 V through a service cut-out having an HBC fuse to BS 1361 Type II, rated at not more than 100 A.
Breaking capacity rating
Use by ‘ordinary persons’
BS 3036 rewireable fuses have a relatively low short-circuit breaking capacity rating, as shown in Table 1. This compares with
Some inspectors and designers may have wondered whether the words in bold in the following extract from Regulation 533.1.1.2 SwitchedOn Issue 17 - Summer 2010