In the UK is your Domestic dwelling correctly protected by the SPD fitted in the CU/DB

I've been thinking this would come up.

I've been in contact with BG (luceco) about their "no MCB required" SPD, CUSPDT21N, link here http://www.bgelectrical.uk/uk/circuit-protection/devices/spd

This is what they replied previously 

"Thank you for your detailed enquiry regarding the SPD in question. We appreciate your close attention to standards and your commitment to electrical safety. Please see our responses to your queries below:

1. Labelling with 100A gG Fuse Symbol Without Internal Fuse

The symbol and marking indicating "100A gG" on the front of the SPD refer not to an internal fuse, but to the recommended rating and type of external backup overcurrent protection device (OCPD) to be used in coordination with the SPD. This marking is a common industry practice and intended to guide installers on the appropriate external fuse rating. We agree that clearer labelling may help avoid any confusion, and your feedback will be passed on to the product design team for review.

2. Recommendation for 100A gG Fuse and Its Location

You're correct that the SPD is designed for direct connection to the busbar and does not include an internal fuse. The 100A gG fuse recommendation is based on coordination tests to ensure safe disconnection under fault conditions. This fuse is expected to be located upstream, typically at the main switch or upstream protective device serving the consumer unit.

Although it's termed as a "recommendation," this is in line with BS EN 61643-11, which allows the manufacturer to state coordination requirements based on tested scenarios. The installer must ensure compliance with BS 7671 and provide appropriate protection based on the installation context.

3. Discrimination with DNO Cut-Out Fuse

You raise a valid concern. Indeed, discrimination between a 100A gG fuse and a 100A BS 1361 cut-out fuse is unlikely due to similar time-current characteristics. In the event of SPD failure and a consequential fault, it is possible that either fuse could operate.

BS 7671:2018+A2:2022 Regulation 534.4.5.2 advises that failure of the SPD should not affect the continuity of supply to the installation. However, if only upstream protection is provided (e.g., by the DNO fuse or a main fuse), total disconnection is possible, which could contradict this guidance. To comply with 534.4.5.2 and 534.4.6, dedicated external OCPDs (such as a miniature circuit breaker or fuse) for the SPD may still be the preferable solution for maintaining supply continuity and fault protection integrity.

4. Compliance with 534.4.5.1 and SPD Protection Design

Regulation 534.4.5.1 does state that SPDs shall be protected by an internal or external OCPD. In the absence of an internal device, an external OCPD is indeed required, not merely recommended. The SPD in question assumes that an upstream OCPD is already present in the installation or will be provided. If not, the installation would not be compliant with BS 7671, as you’ve rightly pointed out.

We acknowledge your observation that using a dedicated MCB for the SPD simplifies compliance and enhances selectivity. While direct-to-busbar SPDs are often designed for convenience and compact installations, care must be taken to ensure proper external protection is in place.

Conclusion

To summarize:

The "100A gG" marking is indicative of the required external backup protection, not an internal component.

A dedicated external OCPD is required for full compliance with BS 7671.

Where selectivity with the DNO fuse cannot be guaranteed, installation of a dedicated protective device local to the SPD is strongly advised.

Your suggestion regarding retaining a dedicated MCB is technically sound and often preferable in practice.

Thank you again for your thoughtful questions. Please don’t hesitate to reach out for further clarification or to discuss a specific installation scenario.

Kind regards,

Support Team"

Edit; I haven't fitted one, but a friend has bought a board with this fitted. After showing him this reply he phoned niceic tech, who basically asked what protects the main switch and circuit breakers? He said the DNO fuse. They said exactly, go ahead and fit it and it will be up to the manufacturer if it goes wrong. Anecdotal I know but poor advice surely.

So the Datasheet for BG states

No MCB required

However there support team reply back with

A dedicated external OCPD is required for full compliance with BS 7671.

I think this might be one for Graham.K to advise

I can understand their point.  In the UK, we fit 6kA breakers in consumer units, and rely on the supply fuse to cover those extremely rare events when the fault current is more than 6kA.  It would be rather silly saying that we have to add an additional 16kA breaker before every 6kA rated one.

Similarly, these SPDs contain their own built-in fuse.  So in the vast majority of cases, that's enough.  But there's always the possibility that something could go catastrophically wrong, in which case, the supply fuse would blow.

I can understand their point.  In the UK, we fit 6kA breakers in consumer units, and rely on the supply fuse to cover those extremely rare events when the fault current is more than 6kA.  It would be rather silly saying that we have to add an additional 16kA breaker before every 6kA rated one.

Similarly, these SPDs contain their own built-in fuse.  So in the vast majority of cases, that's enough.  But there's always the possibility that something could go catastrophically wrong, in which case, the supply fuse would blow.

I am not sure that SPDs contain their own built-in fuse in UK models

Simon Barker said:

But there's always the possibility that something could go catastrophically wrong, in which case, the supply fuse would blow.

A 100amp BS1361 fuse can handle 1.45 x rated value for up to 1 hour. so that's 145A. BS7671 is for protection of persons, livestock, and property from electrical hazards. This includes protection against electric shock, fire due to electrical faults, and thermal effects

Not sure under what conditions an SPD, working or failing would carry 140 plus amps for any significant time at all, or even 1.4 amps for that matter would be perhaps tens of seconds, without doing itself and anything along side a serious injury.

There is no magical dissipation capability inside an SPD - the peak power can be very high, but the total energy and so the time it handle it for and stay cool is very limited.

Continuous dissipation in an MCB sized lump is limited by overheating considerations to  single figure watts. As far as the SPD is concerned,  all the surge energy during the '25kA' rating test is delivered during a pulse of nominal 8 microsecond rise 20 microsecond decay time respectively. Tens of thousands of amps times hundreds of volts times tens of microseconds is still only tens of joules.

This zap you cold repeat every few seconds, giving the device time to cool between strikes, until it failed.

Mike.

Pretty sure that the OCPD is there for when it fails. A manufacturer once confirmed this to me. If it fails closed circuit then large amounts of energy will be flowing to earth or neutral. Someone mentioned in another recent SPD thread that MOVs tend to fail closed circuit. I believe it is for this reason that bs7671 obliges us to fit a dedicated OCPD for an SPD, else the DNO fuse will blow and darkness will prevail.