You wouldn't install a wet sprinkler protection system in an electrical sub-station, you would use an inert gas system, if you needed protection in there at all. Personally I've not come across a wet system in a sub-station.

My career after fire brigade started as FPO in a General Motors auto electronics factory with 100% wet sprinkler protection, except for the Sub Stations which were covered by CO2.

Even the petrol engine test rooms were wet sprinkler protected. The theory was if you apply enough water it will extinguish any fire.

You wouldn't put a "traditional" wet sprinkler system into an HV substation.

Are you actually thinking of sub-stations, or switch rooms?

In all reality, most of the HV in the UK is run by the DNO & National Grid.
It is rare for there to be much HV around a building unless it is HUGE, then you would have a nominated SAP who would advice on the HV.
Remember whilst 230V will kill, it is unlikely that anyone will survive an HV sock, and an "open casket funeral" won't be an option IYKWIM.

Sub-stations are synonymous with HV/HV, or HV/LV transformers.

If you are talking about LV switchrooms, then this is a different scenario.

I would not be keen on "traditional" wet sprinklers in one of these either, but, it could be done, "traditional" wes sprinklers on HV, hmm, I'd rather not be around TBH.

If you do have substations on site, & they are yours you need to rope in your SAP.
I am very surprised that you have not already asked them?

Whilst 230/400 V LV, can be survived HV is another animal completely, it takes no prisoners and can jump a LARGE gap.

Just look at the separation distances for HV OH lines to get a clue.

The other issue you would have in the event of an activation, HV is often fitted with auto-reclosers, so you could have pooled water, persons entering this and all of a sudden you have a re-close, and you have perhaps a 1kV step potential in the pooled water.
Fire Service PPE is not going to save them!
They will be dead.

We really need to know if you are on about HV/HV, HV/LV substations or LV switch rooms, there is a BIG difference

Most new HV in buildings these days is encapsulated where possible anyway.
Cast resin transformers rather than oil filled etc.
Not that they can't catch fire.
Also, if you do have HV on site, you need to understand whose it is.
If it belongs to the DNO, then they will have a BIG say in what you do, as if you take out one of their RMU's they will not be happy bunnies, and along with the other businesses you put off supply, who will sue you for losses, the DNO will charge you for re-instatement.

You need to understand what you are protecting, why, and what against.
What does the design FMEA say?
It is the design FMEA that has taken you down this route?

To simply blindly specify wet sprinklers for electrical equipment takes a lot more thought than it may at first seem.

In my experience the design is normally covered by a competent fire sprinkler suppression company and the insurers and not the local health and safety bod. I would suggest Sapper.D that you take advice from a competent source. That's not to say those giving advice here are not competent but you should have someone TAKE A LOOK AT THE SITE, hazards and risks and advise and then run it by the insurers.......IMHO.

Regards