Air Break Switch 1

[Danilo917]

Gents,

Just a general inquiry that i hope would be worthy of discussion..is an Air Break Switch rated at 12/24kV considered as protection device for short circuit or it is just an isolator and meant to be damaged during fault condition?

thanks,
Danilo

 

[Marmite]

An air break switch is not a protection device. Usually they are fault making and load breaking. The actual rating will depend largely on the contact arrangements and whether they are plain break, fitted with arcing horns or interrupter heads. It shouldn't be damaged by a through fault if the current is within the short circuit rating. If an air break switch opens under fault conditions it will be sacrificed, and will need to be replaced. I've seen 132kV disconnectors used in sacrificial mode to protect a 132/33kV Tx when the normal intertripping was depleted but I don't think anyone would do that these days as the cost of circuit breakers and circuit switchers has dropped significantly in proportion to the cost of the transformer.
Regards
Marmite

 

[Sn00ze]

We use a (b) contact from the feeder breaker to block the A/B switch from operationg. Also, some clients straight up ask to disable the push button/manual operation for the A/Bs because of human error.

 

[Sn00ze]

I think the wording "permissive" is better description instead of "block"

-sn0

 

[stevenal]

"Usually they are fault making and load breaking." Perhaps you meant to type "non-load breaking"? You need to specifically spec out load breaking capability if needed, it is not the norm.

 

[Danilo917]

Gents
thanks for your replies..to quote Marmite ' It shouldn't be damaged by a through fault if the current is within the short circuit rating.'..the ABS assembly im referring here is rated at short circuit fault of 25kA/1sec. My understanding of this is ,when the fault current exceeds 25kA and more than 1 sec, the ABS will start to fail..is this correct? Also, i assumed that any electrical equipment that functions as isolator is graded from the source's fault level,so it should serve as a 'short circuit protection device'.

The load breaking function of this ABS is optional,so i dont expect this to operate under overload condition but I expect this to operate or fail under short circuit condition which makes this a 'protection device'.Please correct me if im wrong.

- Danilo

 

[Marmite]

The first point I would make is that you should never install switchgear in a part of the network where it will be overstressed by excessive short circuit fault current. To do so is irresponsible and negligent.
The rating is assigned by the manufacturer and substantiated by type testing. That doesn't mean that it will fail if it is exposed to >25kA for >1 sec, it just means that there is no guarantee that it won't fail, because you are exceeding the parameters which it was type tested and rated for. The length of time the switchgear is exposed to the fault current increase heating, the magnitude of the current causes large electromagnetic forces.

I now wish I hadn't mentioned the use of an air break switch in sacrificial mode, because I think I have misled you. That was a very rare application which came into play only when both channels of a duplicated intertrip scheme were out of service, and I would not advocate it in any way.

I don't know whether we have a bit of a language problem here, but an air break switch is never a short circuit protection device, neither is it an overload protection device, and causing it to fail catastrophically by exposing it to excessive short circuit current like some kind of fuse, does not make it a protective device either.

Air break switches are fairly crude devices, only a couple of steps removed from the big knife switches beloved of Frankenstein movies.

They are used out on the network for reconfiguration of the system, and for providing points of safety isolation to permit work on items of high voltage plant, or overhead lines. They provide a visible isolation which is required to make part of the system safe for staff to work on it under the electrical safety rules of many jurisdictions.

Regards
Marmite

 

[Danilo917]

Thanks Marmite for your valuable input.I see your points clearly and i believe theres no language problem either..im just hypothetically figuring out what will happen if the air break switch is expossed to a fault level more than it can handlle..i got your point how it reacts to massive fault level..But as i said also, an ABS with LBS function DOES exist.it functions similar to automated LBS in the form of ABS (side break) remote or scada operated.it can break a load up to 630A at12/24/36 kv..so i can say that it is also an overload protection. Im maybe wrong to assume that its a short ckt protection device thats why im getting an expert's opinion.
-Danilo

 

[rcw retired EE]

A load break switch is not an overcurrent or short circuit protection device. The switch has no mechanism to initiate opening when current exceeds some setpoint. It can be used to switch load, if it is load break rated.

But let's assume that you still want to automatically open the switch when current is > 600A. An overcurrent relay could be wired to tell the switch to open on overcurrent. The motor would then open the switch in about 2-10 seconds depending on the design, gearing ,etc. Sounds good, right? WRONG!

"Overcurrents >600 Amps" includes fault currents. What if there was a 25,000A fault? The switch would still try to open and destruct itself and possibly kill someone in the process.

Your reasoning that everything is graded from the source can also be applied to wires and cable which are also designed for the system short circuit level and will open circuit at some high current. Following this logic, we could save the cost of breakers and switches and just rely on the wires to melt down.

Switches are isolating devices that may be applied within their ratings as load current interrupting equipment with careful consideration of what happens during faults. They are not short circuit or overload protection. Leave that to breakers and fuses.

 

[stevenal]

"...but I expect this to operate or fail under short circuit condition which makes this a 'protection device'."

Speaking to the fail side of this statement, I see you are expecting a switch, not designed as such, to act as a fuse. Fuses are protective devices, designed not only to melt but to also snuff out the resulting arc. When your switch is exposed to a fault over its rating, it is likely to melt. But if the metallic path melts away, the arc remains. This arc will continue to carry the fault current until an actual protective device operates.