Thermal overloads / oil filled contactors.

[RF Lighting]

I'm still on with a periodic at the moment, and we were feeling brave today and decided to have a go at tackling the main switch room.

Many of the DBs are fed from here, and it appears that the only over current protection for the submains cables is provided by these contactors.

I have encountered them once or twice before, but I am not fully familiar with their operation.

We are trying to determine their rated tripping current, but AFAIK they are adjustable, and it's proving a bit arkward.

These are all single phase supplies, and at a guess, we think the overloads are set for about 50A

Here are the adjusters. They are the brass bits which normally sit in a pot of mineral oil, but the cover is missing from a couple. The brass bits can be wound in and out, and there is a scale stamped ono them, but it unfortunatly goes from A - G rather than the actual current rating.

Is the lack of oil likely to affect the tripping characteristics too?


There are some other contactors in there as well. One is about twice the size, and is stamped at 130A SP. There are some bigger TP ones as well.



Sorry about the photo qualtiy. I forgot my camera again and had to use my phone.

 

[flameport]

Not too clear in the pictures, but these things usually work like this:

The brass tubes connect to rods, which in turn have an electromagnet (coil) above.
If the current through the coil reaches a certain value, the rod is pulled up, the contacts trip out and disconnect the circuit. (Similar to the magnetic part of modern circuit breakers)
The oil stops the rods/tubes moving too quickly, so provides a form of time delay, so short periods of overload won't trip. The A-G scale is probably various levels of time delay, rather than the trip current.

No oil will certainly affect the operation, could either cause tripping too quickly, or possibly not at all due to corrosion/dirt getting in there.

 

[johno12345]

What date would you put on that? We have some similar stuff in our derelict powerhouse I thought it was DC stuff. sadly its all been vandalised. Seems a bit excessive for sp supplies. I think I have the remains of a mercury arc rectifier. Loads of wooden CT transformers too, Ill have a better look in there sometime, bit dangerous, unguarded flooded pits and all

 

[1john]

very interesting to see and read about for me who has only done house bashing and light commercial/industrial work.

 

[wingcoax]

I would highly reccommend you tell customer that if the equipment is not maintained he is going to get a fail on it. With no oil in the dashpot the O/L is either going to operate at low current or not at all as the piston will have dried out and seized. BE AWARE some of the old dashpot pistons had asbestos washers on.

 

[Spark123]

I havn't come across this type of dashpot circuit breaker? before, I have come across dashpot overloads for large motors. The reason for there not being a specific current rating may be to do with the differing viscosities of oil, a thicker oil provides more damping hence the CB will have different characteristics to a thinner oil. They will require regular maintenance to ensure the oil is OK too.
What make is it?

 

[NotHimAgain]

Read this http://www.hse.gov.uk/fod/infodocs/483_27.pdf

Note Paragraph 5 - this is why sub-station doors usually used to open outwards .

It allows the pressure wave to be released and blows the engineers body parts outside where they are easier to pick up.

 

[ban-all-sheds]

Ah yes - the ever entertaining BLEVE.

 

[wingcoax]

I think you will find somewhere there will be a chart of the settings. I have not found different oils used. Just winding the pot up or down.

 

[sparkyy]

The brass tubes are oil filled dash pots. There is a piston inside the dash pot with a rod attached to it, the rod passes through a coil at the top of the switch, the more current flowing through the switch the stronger the magnetic field in the coil and the further up the rod is pulled until it hits the trip bar. The further the piston is in the tube the more oil it has to travel through which has the effect of slowing it down changing the tripping characteristics, allowing for inrush current etc. We always used one type of oil, from a bottle marked silicon oil. The current rating of the switches were stamped on the front of the switches we had.

 

[RF Lighting]

Thanks chaps, some very useful information as always.

Nice pic sparkyy. Is that lot still in service? It's in a lot better condition than ours!

Do you know what make those contactors are? They look the same as ours. I'm sure the name begins with an E

 

[sparkyy]

They are Ellison breakers. The one's in the picture were just a few of dozens in the textile mill I used to work in. It closed in 2004 and all the switch gear was scrapped a couple of years ago, there was some nice old stuff. Some more for the history buffs.

 

[johno12345]

that looks just like our powerhouse but ours isnt as clean flooded, full of who knows what, its sad really. Its hard to believe how they had mosaic floors and glazed brick walls, decorated better than modern offices

 

[RF Lighting]

It's a proper work of art is that.

Our's sounds like johno's. All rusty, covered in all sorts, leaky roof and full of discarded PFC caps etc.

I'll try and remember my camera next time I'm down there. There are some nice 300A switch fuses feeding PILCs to busbar trunkings etc.

The only two sites I've come across these on were also former textile mills. Coincidence?

 

[ban-all-sheds]

that looks just like our powerhouse but ours isnt as clean flooded, full of who knows what, its sad really. Its hard to believe how they had mosaic floors and glazed brick walls, decorated better than modern offices

After it shut down, but before they half demolished it and left the rest to rot I went on a tour around Battersea power station. Magnificent quality of materials in the control room.