Motorised Knife Switch.

I maintain the (BirdBrain) floating normally low current two wire line irrigation control systems in a very large plant nursery.
I have remedied the earth leakage fault and lightning protections groundings on a particular system that was suffering recurrent blow ups at the office building controller end.

These systems are able to be remotely controlled by smartphone.
It is proposed that some kind of motorised knife switch would be prudent to be included in the systems to isolate the master controllers during lightning storms.

These switchers could/should be connected in series with the two wire lines at the cable entry point into the building, externally and immediately adjacent to a new/verified three long earthing stake earth ground connection.

Does anybody have recommendations/experience on some kind of electrically controlled switching that is suitable for this application of robustly isolating the master controllers from two wire line HV lightning induced spikes/surges ?.

Thanks. Dan.

knife switch.jpg
 
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scott wurcer

Disabled Account
2004-01-26 3:03 pm
Belmont MA
I assume you want the knife switch because it is inherently not damaged by lightning unless a direct hit is enough to vaporize it? There are solid state relays like Crydom that come in very high capacity which you could precede by a gas discharge lightning arrester. This would protect from all but a direct hit. The Crydoms use only a single logic load for any logic family.
 
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Yeah, we want idiot proof total isolation.
There are dents in the dry-wall opposite from a controller being blown clean across the room.....this property is prone to lightning hits every autumn and the energy/voltage available is stupendous.

Contactors have been tried but lightning has jumped/arced straight across the terminals as above.
A SSR might not be much/any better.

The new earth stake/arrestors installation directly outside the building might be sufficient but we don't trust it, given prior experience.
The central controllers are worth/cost 10K FFS, and the crops worth millions....a few hundred for remotely controlled switching/isolation is well within budget !.

Thanks for the suggestion.
I will think it through a bit further......maybe find/bespoke something useful.

Dan.
 
Last edited:

scott wurcer

Disabled Account
2004-01-26 3:03 pm
Belmont MA
Yeah, we want idiot proof total isolation.
There are dents in the dry-wall opposite from a controller being blown clean across the room.....this property is prone to lightning hits every autumn and the energy/voltage available is stupendous.

Contactors have been tried but lightning has jumped/arced straight across the terminals as above.
A SSR might not be much/any better.

The new earth stake/arrestors installation directly outside the building might be sufficient but we don't trust it, given prior experience.
The central controllers are worth/cost 10K FFS, and the crops worth millions....a few hundred for remotely controlled switching/isolation is well within budget !.

Thanks for the suggestion.
I will think it through a bit further......maybe find/bespoke something useful.

Dan.

If you dig you might find some grid cutoff switches but I suspect they are in the 10's of thousands of dollars.
 
Original Install By A Goose....

Haha, yes I am aware of those......massive overkill but would be impressive.

Master controller output and two wire line cannot be shorted or grounded in normal state or protection state.
I'm thinking toward two CO contactor/relay assemblies in series with the link wire changing over to grounded state......that and a bunch more transorbs.

Dan.