Selection of Disconnect Switch needed for our 480V AHU Panel...

[dsdelarosajr]

Needed your help in selecting a 3KA switch disconnectors to be used as maintenance and transfer switches in distribution boards for our AHU System with Rated Load of 128.5 KW and Rated Current of 163.5 Amps.. Our previous Switch Disconnector was a Siemens - 3KA7122-4AA00, 4-poles with a rated amperes of 200A but already burnt (hence, we don't know the caused of overheating)... Hoping for somebody to help us, thanks & God Bless!

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[ron]

Siemens makes good products, along with Schneider, ABB and Eaton.

Be sure it carries the horsepower rating associated with your AHU.

 

[Jraef]

Siemens makes good products, along with Schneider, ABB and Eaton.

Be sure it carries the horsepower rating associated with your AHU.

Yes, I suspect this is at the heart of your issue. That switch only has a utilization category of AC-21, which is for resistive loads only. It has no motor loading rating listed for it at all, or if it does, Siemens doesn’t even bother to list it. Opening up an inductive load such as a 128.5kW motor, while running, is very hard on switching devices. So in selecting the device you must do one of two things:

1. Select a switching device with a kW motor load rating at or greater than your motor load, or

2. ENSURE in your design that the switch is used only for isolation, which means another properly rated device, such as a contactor, will ALWAYS open the motor circuit BEFORE the switch main contacts open. One thing I see happen is that the system may have originally been designed that way, but over the years modifications and repairs have been made. The result is that the contractor is gone, or the coil circuit is no longer tied to six contacts in the isolation switch (or those contacts have welded), and now the isolation switch is opening under load.

 

[dsdelarosajr]

Yes, I suspect this is at the heart of your issue. That switch only has a utilization category of AC-21, which is for resistive loads only. It has no motor loading rating listed for it at all, or if it does, Siemens doesn’t even bother to list it. Opening up an inductive load such as a 128.5kW motor, while running, is very hard on switching devices. So in selecting the device you must do one of two things:

1. Select a switching device with a kW motor load rating at or greater than your motor load, or

2. ENSURE in your design that the switch is used only for isolation, which means another properly rated device, such as a contactor, will ALWAYS open the motor circuit BEFORE the switch main contacts open. One thing I see happen is that the system may have originally been designed that way, but over the years modifications and repairs have been made. The result is that the contractor is gone, or the coil circuit is no longer tied to six contacts in the isolation switch (or those contacts have welded), and now the isolation switch is opening under load.

actually the circuit have a CB and Protection Relays but it not been actuated during the burnt-out (actual temperature measured on disconnect switch was 299deg.C using our FLIR thermal scanner), said disconnect was not been used to open when its CB was closed...

 

[topgone]

actually the circuit have a CB and Protection Relays but it not been actuated during the burnt-out (actual temperature measured on disconnect switch was 299deg.C using our FLIR thermal scanner), said disconnect was not been used to open when its CB was closed...

Disconnects fail when switch contacts develop verdigris when left unchecked for a long time. That's just the nature copper contacts. As the switch develops high contact resistance, there will be heat developed, the copper contacts melt (even including the bakelite switch contact supports). Since you mentioned you have FLIR thermal scanner, it would best for you to develop a regular thermal scanning of all the disconnects switches in your place as well as CB terminals/ busbar bolts. Do scanning with low lighting levels in the area to prevent lights from interfering with your job.

 

[dsdelarosajr]

Disconnects fail when switch contacts develop verdigris when left unchecked for a long time. That's just the nature copper contacts. As the switch develops high contact resistance, there will be heat developed, the copper contacts melt (even including the bakelite switch contact supports). Since you mentioned you have FLIR thermal scanner, it would best for you to develop a regular thermal scanning of all the disconnects switches in your place as well as CB terminals/ busbar bolts. Do scanning with low lighting levels in the area to prevent lights from interfering with your job.

Thank you very much, it is probably what happened considering our plant is a geothermal power plant and have a corrosive environment (presence of H2S)...