NAPIT’s Don Holmes looks at the two different circuit-breaker standards, and provides clarification on the differences between them.
Two standards that specify requirements for low-voltage circuit-breakers (cbs) have been causing confusion for some time among designers and installers of electrical installations. They are BS EN 60898-1 and BS EN 60947-2 and questions are often asked about the difference between them.
This article will try to clarify the situation.
BS EN 60898-1 circuit-breakers
Circuit-breakers to BS-EN 60898-1 are used for household and similar installations and are also commonly found in retail premises, schools and offices. They are intended for use in pollution and humidity-free environments where their operation is by uninstructed persons. They have current ratings from 6 A to 125 A, a maximum rated short- circuit capacity (Icn) of 25 kA and an impulse withstand voltage (Uimp) of 4 kV. They are available with type B, C and D tripping characteristics.
Circuit-breakers to BS EN 60898-1 are safe and easy to use, even after many years without maintenance. They are suitable for Pollution Level 2 and their classifications and applications are shown in Tables 1a, and 1b.
BS EN 60947-2 circuit-breakers
Circuit-breakers to BS-EN 60947-2 are frequently used for industrial applications in utilities and manufacturing industries. They are used to protect power distribution circuits up to 1,000 V AC and 1500 V DC with current ratings of 0.5 A to 6,300 A. They use ‘miniature’ circuit-breakers (cbs), moulded case circuit -breakers (MCCBs) and air circuit- breakers (ACBs).
Circuit-breakers to BS EN 60947-2 are intended for operation by skilled users and they must be maintained. They are suitable for Pollution Level 3, which includes unheated rooms, boiler rooms, industrial and farming areas.
Essential operational reliability
Two requirements that ensure reliability of circuit- breakers are the ultimate breaking capacity (Icu) and service breaking capacity (Ics).
Icu is the maximum short-circuit current that the circuit- breaker can break without damage which might be 6,000 A or 10,000 A and in the case of MCCBs, 200,000 A for specific voltage ratings.
Ics is expressed as a percentage ratio of Icu and gives the maximum short-circuit current a cb can break three times and still function in normal service.
Circuit-breakers are primary circuit protective devices. They are not intended for frequent switching of loads. Infrequent switching of cbs on load is admissible for the purposes of isolation or emergency switching.
Where a more frequent switching is required, such as the switching on and off of banks of luminaires, the manufacturer’s instruction should be followed and preferably an alternative device should be selected.
Complying with both standards
Circuit-breakers to BS EN 60898-1 can also comply with BS EN 60947-2 but the short-circuit breaking capacity of each may be different. Some manufacturers state that their cbs with a short-circuit capacity of say, 10 kA comply with BS EN 60898-1 and the same cb with a short-circuit capacity of 15 kA complies with BS EN 60947-2.
It is convenient when the performance of a cb meets the requirements of both standards and is therefore suitable for residential, industrial and commercial installations.
Can BS EN 60898-1 be used instead of BS EN 60947-2?
Circuit-breakers to BS EN 60898-1 may look identical to those complying with BS EN 60947-2 but they are not necessarily inter-changeable.
For example, the BS EN 60898-1 standard describes B, C and D operating curves with ratio to rated current. But the BS EN 60947-2 standard prescribes an instantaneous tripping release which may be provided with a plus or minus 20% tolerance and adjustable as illustrated in Fig. 1. For this reason, manufacturers provide additional curves K, Z and MA to the B, C and D curves.
A cb designed for Pollution Level 2 conditions would not be suitable for harsh outdoor or humid applications that require Pollution Level 3.
Maximum earth fault loop impedance (Zs) values are tabulated in BS 7671 for cbs to BS EN 60898-1 but the maximum Zs values for cbs to BS EN 60947-2 need to be obtained from the manufacturer.
Manufacturers warn against installing circuit- breakers of one manufacturer as replacements for cbs of another manufacturer, without the necessary verification of performance.
Although cbs from different manufacturers may appear similar, the technical performance, dimensions and terminations are not always compatible.
Assemblies such as distribution boards are validated with specific circuit-breakers installed and these cbs are usually from the same manufacturer as the distribution board. Where cbs made by a different manufacturer are to be installed, verification will have to be undertaken by the manufacturer of the distribution board to BS EN 61439-2 or BS EN 61439-3. Fitting unverified devices will invalidate any verification and the warranty.
Regulation 510.3 of BS 7671 requires that every item of equipment selected and installed must take account of the manufacturer’s instructions.
Any installer who plans to substitute a different cb must obtain authority from the assembly manufacturer to do so in compliance with Regulation 536.4.203. Otherwise, responsibility for the testing and integrity of the compatibility between devices would be down to the contractor. When this is not carried out, there is a probability that, in the event of an accident, fire or other damage, the installer would be accountable under Health and Safety Legislation.
It is important to select the correct type and current rating of circuit- breaker for the environment, circuit and equipment if adequate protection for overcurrent, short-circuit, fault currents and unwanted tripping is to be provided. Manufacturers’ data should be referenced when designing circuits and they should be contacted where additional information or clarification is needed, especially in the case of MCCBs and ACBs.