Air Break Switch
Air Break Switch
(OP)
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
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






RE: Air Break Switch
Regards
Marmite
RE: Air Break Switch
RE: Air Break Switch
-sn0
RE: Air Break Switch
RE: Air Break Switch
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
RE: Air Break Switch
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
RE: Air Break Switch
-Danilo
RE: Air Break Switch
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.
RE: Air Break Switch
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.