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Backup fuse for SPD

03/05/2022 2:47 PM

Hi everyone ,

should there be a backup fuse for Q0.1-SPD and what size? Schematic is attached.

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#1

Re: Backup fuse for SPD

03/05/2022 5:19 PM

There is no electric power to a surge protection device to my knowledge...If you are talking about a secondary surge device, I guess you can have as many as you want, maybe one on the incoming side and one on the load side, but that seems to be overkill, better to test often if you are that concerned...

https://www.ecmweb.com/power-quality-reliability/article/20900951/understanding-surge-protective-device-ratings

https://professional-electrician.com/technical/surge-protection-devices-can-you-check-them-using-a-mft-metrel/

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#2

Re: Backup fuse for SPD

03/06/2022 8:44 AM

Given that circuit breakers and fuses are there to protect the downstream wiring, the answer is yes, and it needs to be compatible with the downstream wiring to the <...SPD...>.

In the UK, BS7671 gives full protocols for this. The same is probably true for national electrical standards in use elsewhere.

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#3
In reply to #2

Re: Backup fuse for SPD

03/07/2022 4:05 PM

...and if it isn't, then it's a significant opportunity for improvement.

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#4

Re: Backup fuse for SPD

03/09/2022 3:28 PM
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#5

Re: Backup fuse for SPD

03/17/2022 1:39 PM

I see many people do it this way and have been told that it’s because the SPD is not a “load”, ergo the circuit is not a “branch”. I have always disagreed, but have not pursued proving it further because of the following:

I would never have an SPD that is not able to be disconnected from the line to be replaced. SPDs often sacrifice themselves in the line of duty, so need replacement periodically. In the scenario shown, the only way to replace them would be to disconnect the main incoming power, shutting down everything. So I always insist on a disconnect ahead of the SPD, and the cost of adding fuses or making that a circuit breaker is not worth arguing about. In addition, sometimes when an SPD dispatches a surge, it does do “with extreme prejudice” as they say in movies, meaning the act involves a minor explosive event, which can lead to causing a flashover that could then end up causing the Main protective device to open, shutting everything down. Having a separate Short Circuit Protective Device ahead of the SPD can help isolate the fault.

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