What is this symbol?

[unclebob]

Hi all!

Please refer to the attachment.

This is a 6600 V switchgear, built in the 40's. In it, there are neon ground lamps witch appears to be inductively coupled to the bars. We're wandering what this capacitor-like symbol represents? It is not possible for us to inspect these devices without a total shutdown.

Anybody here ever seen this?

 

[marks1080]

Looks like a potential transformer. Hard to tell without seeing what the rest of that circuit is connected to.

 

[jghrist]

Looks like a potential transformer. Hard to tell without seeing what the rest of that circuit is connected to.

I don't think the OP means the highlighted VTs. I think he means the neon grounding light connection to the bus. I suspect some capacitive connection. It would have to be something with a set impedance so that with no ground on the bus, the voltage will be zero across the lamps.

 

[opmgr1]

I think it is some contact type connection.

 

[Skogsgurra]

It looks like this is a system with a very high prospective short-circuit current. There are resistors in series with the fuses for the VT:s and I suspect that the capacitor-like symbols are exactly that, capacitors. I have seen feed-through capacitors made from an isolating tube pushed onto and fixed to the busbar. Around that isolating tube, another tube that was used to pick up enough power to light neon lamps. Current limiting and short-circuit protection in one simple "device".

HRC fuses are expensive and if you can get by using this "device" - why not?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[burnt2x]

Clip-type tap, not bolted! Just my guess.

 

[controlsdude]

this reminds me of a spark gap I used in radar modulators. basically its a fuse like Gunnar pointed out.

Radar use in a modulator
The spark Gap when there was a high current level ionized the gas inside it and arced across the gap and this avoided destroying the modulator when there was a high current draw.

When they ionize one time you have to replace them. Or they just keep arcing but at a lower current.

This must of been a painful experience only reason i remember this.

 

[Skogsgurra]

Hey burnt2x!

I think I know how you got your handle! Clipping to those bus-bars with no fuse would probably change your handle to burnt3x or burnt4ever!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[WoodrowJWeen]

This idea of neon lamps capacitively coupled to medium voltage bus or terminals may be old but is still used. Worked on a project in Washington, DC area several years ago and PEPCO (local utility there) required these on a 13.2kV switchboard. These were installed inside the terminal compartment as a safety feature so that their linemen could see immediately upon opening the rear door whether there was voltage there or not.

 

[WhyDoYouAsk]

The neons might fire at 65v, and they need 1mA, so you can figure out how much capacitance these couplers must have (at 60Hz).

 

[isquaredr]

I have seen this type of capacitive coupling, beware removing the neon bulbs with the primary live can give rise to high voltage on those secondary connections. The capacitor (I think) is achieved by wrapping foil in with the paper insulated bushing.

 

[cranky108]

Those look simular to condencer bushings.
In some older equipment in the wraping of the insulation of the bushings, they would insert a metal foil and use it as a potential source.
The bushing acted as a condencer voltage divider.
Condencer bushings work somewhat well as long the secondary load dosen't change.

 

[mlhspike]

It's a condenser bushing that will limit the current to the Neon light. As above the voltage is not limited so if you remove the neon lamp form the circuit you will get zapped +. The Lights on the other hand are a old time passive ground detector circuit, it is relay only reliable for low grounds as loading on the phases can change intensity of the neon lights.

 

[burnt2x]

Correct me if I am wrong but I was told neon lights glow only when subjected to 6000 volts up! The special wires are insulated 7500 volts and on 120 VAC source, you need a ballast to up the voltage to that neon glow-voltage. That's how Vegas got so nice with this things! The problem was how to limit the current to within 60 mA, hence drop resistors are inserted.

Gunnar, yep, twice is perhaps all that I could handle.

 

[davidbeach]

I've seen neon glow lights that work just fine at 120V and no means of stepping up the voltage. You need so many volts per unit length of neon between the two electrodes for it to conduct; 1/8 inch needs far less voltage than 20-30 feet.

 

[burnt2x]

Thanks David. Forgot miniaturization can do things that were impossible way back in the 40's.

 

[unclebob]

Thanks for all your inputs. I'll try to get a visual at the next shutdown.

 

[controlsdude]

I think everyone is right, could be different applications for these type of tubes. Depends on how they are manufactured. Here is a write up that explains a lot of what we are talking about mostly electrical discharges.

http://mysite.du.edu/~jcalvert/phys/dischg.htm#Appl