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Grey plastic covers over electrical cabinets
- Thread starter theageofthetra
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TheJRB
Established Member
Just to add another picture, here's one on the left of this photo I took at Pluckley last month. I wondered what it was at the time!
Elecman
Established Member
It can be very difficult to get a good earth connection of a low enough impedance value to guarantee that any fault voltage at the cabinet can be lower than 50 volts which is the accepted safe value
dysonsphere
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Indeed especially on made up ground like platforms but cant see it being a problem in open country side, or could it also be the parts of the cabinet arnt bonded internally, ie the doors etc.
edwin_m
Veteran Member
Considering that it is probably easier to earth a cabinet than to put a tent over it, there must be some reason why they didn't do that. I assume the cabinets were designed to be "floating" (electrically speaking) because of the risk that "earth faults" can lead to wrong side failures.
Have any of these tents been noted on 25kV lines? On these routes everything should be earthed anyway.
brianthegiant
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Getting a low earth impedance can be very challenging depending on the ground conditions. Slate has very high resistance. My guess is that railway embankments on clean ballast also have high resistance as they're relatively dry.
The ground resistance is less of a problem if you have a network of electrodes so the total area of your electrode field is large, for a single electrode it is more challenging.
Elecman
Established Member
Indeed so the requirement is to space the electrodes at a distance of 1.25 times their length to avoid overlapping resistance gradients so can take many rods to achieve even a less than 200 ohm earth in bad locations , the actual resistance required may be much less than that value.
Deepgreen
Established Member
At Betchworth, one of four cabinets adjacent to the EB platform entrance path has such a cover. One of the other cabinets has this on it!
Many assets associated with signalling has an IT Earth system. That is, the earth is connected to ground via a high ohm resistor. Technically it is earthed, but dangerous voltages can happily exist with minimal fault current.
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