BS 3871 Miniature Circuit Breakers (MCBs) were widely used in electrical installations until the standard was superseded by BS EN 60898 in the early 1990s. While these older MCBs can remain in service if they continue to function correctly, it’s important to understand their limitations, especially concerning their use as isolation devices.
Suitability for Isolation:
The BS 3871 standard did not include specific requirements for isolation capabilities. Consequently, MCBs manufactured to this standard may not provide a positive air gap with adequate clearance distances to ensure effective isolation. As noted in the BEAMA Guide to Low Voltage Circuit-Breaker Standards:
“Circuit-breakers conforming to BS EN 60898 series are suitable for isolation. However, the earlier standard BS 3871 did not contain verification or marking requirements for isolation.”
Therefore, unless a BS 3871 MCB has been specifically tested and marked for isolation by the manufacturer, it should not be assumed to be suitable for that purpose.
Regulatory Considerations:
The IET Wiring Regulations (BS 7671) provide guidance on devices suitable for isolation. Table 537.4 of BS 7671:2018 Amendment 2:2022 lists devices recognised for isolation purposes, but it does not reference BS 3871 MCBs. Additionally, Regulation 537.2.1 states:
“Devices for isolation shall be of a type for which the isolation function is explicitly recognised by the relevant product standard or as identified in Table 537.4.”
Given that BS 3871 MCBs are not recognised in this context, their use as isolation devices would not comply with current standards.
Practical Implications:
While BS 3871 MCBs can remain in service for overcurrent protection if they are functioning correctly, they should not be relied upon for isolation purposes unless explicitly marked as suitable by the manufacturer. For new installations or when modifications are made to existing circuits, it’s advisable to use devices that comply with current standards, such as those conforming to BS EN 60898, to ensure both safety and compliance.
In summary, while you can continue to use existing BS 3871 MCBs for overcurrent protection, they are generally not suitable for use as isolation devices. For isolation purposes, it’s recommended to use devices that meet current standards and are explicitly designed and tested for that function.
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