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Guide to Low Voltage Circuit-Breakers Standards

In accordance with BS EN 60898-1, BS EN 60898-2 and BS EN 60947-2

Circuit-breaker substitution in assemblies 


Recommended practice warns against the practice of installing circuit-breakers (e.g. MCCBs, MCBs) of one manufacturer as replacements or substitutions for devices of another manufacturer, without the necessary verification of performance. Assemblies such as consumer units, distribution boards and panelboards are verified with specific devices installed; these devices are more often than not from the same manufacturer as the assembly. 





Verification will have been undertaken, by the ASSEMBLY manufacturer, to BS EN 61439-2 or BS EN 61439-3 (formerly BS EN 60439-1 and BS EN 60439-3 respectively). Substituting devices not verified by the assembly manufacturer invalidates any testing/verification and warranty. 

BS 7671 puts the specific responsibility on the installer; regulation 510.3 requires that the installer takes into account the manufacturer’s instructions in regard to devices fitted. It is, therefore, the responsibility of the installer who plans to substitute a device, for whatever reason, to a) obtain authority from the assembly manufacturer or b) undertake appropriate verification to ensure conformity with the current BS EN standard. If this is not carried out then there is a probability that, in the event of accident, a fire or other damage, the installer would be accountable under Health and Safety legislation. Although devices from different manufacturers may appear similar, the technical performance, dimensions, and terminations are not necessarily compatible. 



Functional switching with Circuit-breakers 

Circuit-breakers and RCBOs are primarily circuit-protective devices and, as such, they are
not intended for frequent load switching. Infrequent switching of circuit-breakers on-load is admissible for the purposes of isolation or emergency switching. For a more frequent duty the number of operations and load characteristics according to the manufacturer’s instructions should be taken into account. Preferably an alternative device should be selected (See Table 53.4 of BS 7671) e.g. a contractor to BS EN 60947-4 or BS EN 61095 or a switch to the BS EN 60669 series.


Coordination of Low voltage Switchgear and Controlgear assemblies with conductors operating at a temperature exceeding 70°C e.g. XLPE 

BS 7671 regulation 512.1.5 requires that “Switchgear, protective devices, accessories and other types of equipment shall not be connected to conductors intended to operate at a temperature exceeding 70°C at the equipment in normal service, unless the equipment manufacturer has confirmed that the equipment is suitable for such conditions”. BS 7671 regulation; 523.1 (note b) requires that “Where a conductor operates at a temperature exceeding 70°C, it shall be ascertained that the equipment connected to the conductor is suitable for the resulting temperature at the connection”. BS 7671 90°C cable tables e.g. Table 4E4, state that; “Where cables in this table are connected to equipment or accessories designed to operate at a temperature not exceeding 70°C, the current ratings given in the equivalent table for 70°C thermoplastic insulated cables (Table 4D4A) must be used (see also Regulation 523.1)”. 

The above specifically applies to low voltage switchgear and controlgear assemblies which include switchboards, panelboards, distribution boards, busbar trunking systems and consumer units. It also applies to wiring accessories which includes wall switches, socket outlets, fused spurs and plugs. The British (BS) and harmonized (BS EN) standards for these products contain test limits that apply to thermoplastic insulation i.e. PVC, and specifically to low-voltage assemblies, where the terminals of the built-in component e.g. MCCBs/MCBs, also contain the terminals for external insulated conductors. 

Unless specified by the manufacturer, conductors operating at a temperature exceeding 70°C are not suitable or safe for use with wiring accessories, low-voltage switchgear and controlgear assemblies. However, 90°C rated cable can be used for external wiring provided the conductor operating temperature does not exceed 70°C i.e. where the electrical design is based on current ratings given in the equivalent table for 70°C thermoplastic insulated cables. Specifically for a low-voltage switchgear and controlgear assemblies, whenever a declaration states that built-in components (e.g. mcbs / mccbs) which also contain the terminals for external insulated conductors, are suitable for conductors operating at a temperature exceeding 70°C, then the components must have been tested in the assembly as part of the appropriate assembly standard. 



Overcurrent coordination of devices from different manufacturers 

Backup protection of a circuit-breaker by use of an SCPD is only accurately verified by test, appropriate data can be obtained from the manufacturer. Selectivity between devices is only precisely established by test; however, guidance for a method of establishing selectivity by a desk study is given below.



Selectivity in the overload zone Circuit-breakers in series (downstream (C1) and upstream (C2)) – selectivity determination by comparison of characteristics. 


Selectivity in the overload zone is verified by comparison of the time/current characteristics, Separation of the characteristics in both the time and current axes ensures selective operation of C1 with respect to C2, in this zone. There will be a tolerance applicable to the characteristics, which should be taken into account. The manufacturer’s data should show a tolerance band or otherwise indicate the tolerance applicable, as required by the product standard. 


Circuit-breaker (C1) with fuse as SCPD 


selectivity determination by comparison of characteristics Selectivity in the overload zone is determined by the comparison of time/current characteristics. Separation of the characteristics in both the time and current axes ensures selective operation of C1 with respect to the fuse, in this zone. There will be a tolerance applicable to the characteristics, which should be taken into account. The manufacturer’s data should show a tolerance band or otherwise indicate the tolerance applicable, as required by the product standard. 



Determination of selectivity in the fault current (short-circuit) zone 


Determination from time/current characteristics, of selectivity between two circuit breakers in the fault current (short-circuit) zone, is limited to the case where C2 has a short-circuit release time-delay function provided by an electronic release.



Circuit-breakers in series (C1 and C2) 


selectivity determination by consideration of peak let-through current In the case where the instantaneous tripping of C2 depends on an electromagnetic effect (i.e.. thermal/magnetic or magnetic-only circuit-breaker) or in the case of an electronic trip unit with an instantaneous release, the minimum level of selectivity between two circuitbreakers in the fault current zone may be determined as follows: Selectivity is assured up to the fault current level at which the peak current let-through of C1 is less than the peak value corresponding to the instantaneous tripping level (Ii) of C2 taking into account the tolerance. 


EXAMPLE: 

C2 = 800 A MCCB; Ii = 8 – 12 kA r.m.s. (10 kA setting ± 20 %); C1 = 125 A MCCB. 

Minimum tripping level of C2 is 8 x 1,414 = 11.3 kA peak. 

Let-through current of C1 at 15 kA r.m.s. prospective, due to the current limitation of C1, is 11 kA peak, from test data. 

Therefore the system is selective to at least 15 kA r.m.s. prospective. 

Note: the selectivity limit obtained by this method will err on the low side and the actual limit determined by test will be significantly higher in most cases. 


Circuit-breaker (C1) with fuse as SCPD 

Selectivity in the fault-current (short-circuit) zone (see 3.11) is determined from the I 2 t characteristics. The selectivity limit current Is is the maximum value at which let-through I 2 t of the circuit-breaker is lower than the pre-arcing I 2 t of the fuse. In the absence of an actual curve the manufacturer’s quoted I 2 t pre-arc value for the fuse is taken. 5.4.4. Determination of selectivity limit current for specific installation conditions Data on selectivity limits may be supplied in tabulated form, graphically or as software media. Data obtained from either a desk study or tests, to this standard, will be based on the prospective fault current level at the incoming device (C2) and assumes that the coordinated devices are in close proximity. In practice the selectivity limit will be influenced by the impedance between the two devices. Therefore, in the practical situation taking the prospective fault current at the downstream circuit-breaker will give a more precise value for the selectivity limit.

This article was extracted from "Guide to Low Voltage Circuit-Breakers Standards" by BEAMA "



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