Fusing for SPD

Thank you Jack for your reply and ideally what you laid out would be correct. But we don't live in an ideal world especially with SPDs. Adding a disconnect switch will degrade the performance of the SPD, so unless necessary for other reasons, I do not recommend using a disconnect switch just for the SPD.

The trick is to protect your gear from the surges and spikes but also to protect your gear when the SPD gives up the ghost. SPD's often fail violently, although the newest UL 3rd edition is making it better, the SPD failures can damage the very gear you are trying to protect.

Therefore, there is a delicate balance between fusing and not fusing. SPD manufacturers should have fusing requirements in their installation instructions. These should be followed. For example, Dehn SPD products typically allow a fuseless installation up to a 125 A service (at or below 50k SCCR). However, beyond the 125 A 50kA SCCR, they require an additional fuse. The reason for the possible fuseless installation is that the SPD has its own internal fusing mechanism that is rated up to 125A, 50k SCCR.

Thanks for listening, I hope this helps.

There are good reasons to install a fuse in series with an SPD. For one, to protect the wiring to and from the SPD from fire upon a short circuit of the SPD. Another is to prevent catastrophic electrical equipment failure (carbon deposits into gear) from an SPD failure.

Depending on the product, manufacturer, the installation site, you might not need to use a fuse. However, there are items you need to check.

What is the Short Circuit current rating of the SPD and what is the available short circuit current of the electrical supply that the SPD is going to be installed into?

The SPD rating should be equal to or greater than the supply. If not a supplemental fuse is necessary.

Second what is the supply side maximum current rating? You should then check the installation instructions for the SPD products you are using as if they are UL listed or recognized devices, they will have information on the fusing requirements.

I can tell you that most fuses when installed in series with an SPD will lessesn its Imax of 40kA. In other words, the fuse will open before the Imax is reached. However, there are surge rated fuses available such as the VSP fuses from Ferraz. You can google the VSP fuses and get the information. A VSP-40 fuse is rated to pass 40kA, 8x20 without opening, it is also rated as a 200kAIC fuse.

I hope this helps.