NAPIT’s Principal Technical Author, Richard Townsend, takes a closer look at selectivity requirements for distribution circuits.
One question that I have been asked numerous times over the years is: “How do I achieve selectivity in a residential property between a service cut-out fuse and a switch-fuse used for a sub-main without limiting the supply capacity considerably?”
This is a common situation concerning consumer units which are designed to be located in a different position to that of the intake position. The electrician understands the supplier/DNO restrictions for consumers meter tail length and rightly chooses to install a switch-fuse.
In this article, I will explain how to achieve selectivity and discuss whether or not it’s actually required.
Selectivity
Previously known as discrimination, selectivity is the ability of the protective device to operate in preference to another device, which is in a series connection configuration. For correct device coordination, short-circuits, overcurrents and residual currents will need to be considered.
Fig 1 shows a scenario where four protective devices are installed in series (identified as A, B, C and D) as well as the position of a fault which has occurred downstream of protective device D. In order to achieve selectivity, only the protective device immediately upstream of the fault should operate.
BS 7671 Part 2 also makes reference to partial selectivity and total selectivity, as follows:
- Partial: Selectivity up to a given overcurrent lower than the breaking capacity of the downstream device.
- Total: Selectivity for all overcurrents up to the value of the breaking capacity of the downstream device.
Regardless of which aspect of coordination is being studied, BS 7671 provides general requirements for selectivity in Regulation group 536.4, as summarised below:
- Desk study utilising product standards and manufacturer’s literature, or
- The use of appropriate software in conjunction with manufacturer’s data, or
- Tests in accordance with the applicable product standards, or
- Manufacturer’s declaration.
The worked example given in this article relates to device coordination between fuses. From this, it can be seen that it’s not just a case of simply stepping down one fuse size to achieve selectivity. The actual characteristics of the devices used will need to be examined.
Coordination between fuses
For the correct coordination between fuses, there are two aspects to consider:
- Selectivity in the case of overloads, and
- Selectivity in the case of short-circuits.
For overloads, selectivity is validated by comparison of the time/current characteristics of the fuses concerned.
We’re looking to ensure that the total operating time of the fuse positioned on the load side is lower than the pre-arcing time of the up-stream fuse on the supply side.
For short-circuits, selectivity is validated by comparison of the I2t values. As a designer, we are looking to achieve a maximum operating I2t value of the fuse positioned on the load side being lower than the minimum pre-arcing I2t value of the fuse on the supply side.
Total selectivity can be achieved for most practical situations, where fuses are of the same utilisation category with a rated current of 16 A and above if a coordination ratio of 1.6:1 or greater is applied.
For example, an upstream fuse rated at 160 A will coordinate with a downstream fuse rated at 100 A.
Worked example:
Using the coordination ratio 1.6:1, what fuse size is required for the switch-fuse, feeding the consumer unit sub-main? The service cut-out fuse is rated at 100 A.
Answer:
100/1.6 = 62.5 A
In order to satisfy the 1.6:1 ratio, the sub-main fuse would need to be rated at 60 A to ensure total selectivity.
Depending on the maximum demand for the installation, it’s likely that limiting the homeowner to 60% of their supply capacity will be unacceptable, especially with today’s trends requiring more loading than ever, for example, by electric vehicles and heat pumps.
So, is selectivity compulsory?
No, not in all cases. Regulation 536.3 refers to coordination requirements and asks that the interaction between devices shall be considered so they do not affect the safety of the installation.
For simple installations such as the example given in this article, there are no safety concerns regarding selectivity requirements not being met. Let’s say you have selected a 100 A fuse for your sub-main supply; under fault conditions, if either fuse blows, there would be a bigger issue to worry about than selectivity.
The requirements of Regulation group 536.4, which outlines the requirements for selectivity, only need to be applied where the designer decides that selectivity is required. Examples may be commercial installations where safety could be affected or processes disrupted.
Of course, if selectivity is called for as part of a specification or BS 7671 states that it shall be provided, it must be. Regulation 711.536.4.1.4 is an example of where selectivity must be provided between RCDs for exhibitions, shows and stands.
Selectivity between RCDs
The same general requirements can be applied as previously mentioned regarding the use of software, manufacturers’ checks, desk studies, etc.
Selectivity, in the case of residual currents, is as follows:
- The upstream RCD should be suitably time delayed, and
- The ratio of upstream to downstream residual current rating should be at least 3:1.
See Fig 2, which illustrates these requirements.
Conclusion
Where selectivity is required in any design, desktop studies using product standards may not provide the most accurate results. The best approach will be contacting the protective device manufacturers, who provide tables based on tested results.
Often manufacturers’ device selectivity figures offer the most accurate and sometimes more advantageous values than those found elsewhere.
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