example large pressure washer.

How does the water (or any other services) get into (or under) the building? Metallic pipes?

  - Andy.

Water services enter in plastic pipe Andy. No Gas or oil pipe work.

It strikes me that this will end up being an installation somewhat outside the normal rule-book.

In your shoes, at the risk of stating the obvious, I'd return to the basics

There are two prongs to attack a shock risk- first  to interconnect things that may be straddled by animals or staff, so that even if there is a fault current flowing the potential victim sees a voltage drop low enough not to be harmful -though as others have said, in an area with pools of water that is likely to have dissolved salts etc and is unlikely to be insulating, exactly how low that voltage needs to be is a bit moot.

Anything in the fault loop that is not the impedance you straddle that keeps the prospective  fault current down is likely to be good. (ELV fittings would be one way, so the driving voltage was lower, double insulated - class 2 where possible would be another, so that there are no faults to earth under most of the more credible fault conditions, transformer isolated supplies etc)

The second prong is to remove supply within a short enough time of any fault that there is a dangerous transient but it is cut short so it does not cause injury or death. This time generally relates to the heartbeat period, and is for humans is 0.2-0.4 seconds depending on the shock current. For animals with smaller faster beating hearts, I'd expect the safe maximum exposure times to be shorter, but I have no reliable reference for exactly how true that is. Now earth fault relays and RCBs can be very fast, so that is easy.

But you do not want to kill power to any more of the installation than you have to as I presume other parts of the zoo, an I have no idea of your case but I am imagining aquarium pumps, incubator  heaters etc so a zoned approach with the hair trigger protection only where really needed may be good.

It is likely that this will not happen overnight and all that is possible is incremental improvements over time-frame of months or years  so thought to the safety where old and new meet is also required. You may not be able to improve the existing that remains, but you should not make it worse by changes elsewhere

How good an earth electrode are the building steels ?  It may be very well earthed indeed ;-) In any case that needs to be connected to the main installation earth somewhere, and with a bit of imagination about the direction of likely voltage slopes.

Mike