Method and circuit breaker for reducing arc flash during maintenance in a low voltage power circuit

Abstract

Maintenance trip functions override the normal or specified trip functions of a low voltage circuit breaker to reduce pickup currents and/or time delays to effect faster trips, which reduces arc energy produced by faults during maintenance or testing on a live circuit breaker.

1 . A method of providing protection against arc flash during maintenance on a low voltage power circuit including a circuit breaker having a specified trip function for responding to a fault, the method comprising: 
 overriding the specified trip function with a maintenance trip function that results in reduced arc energy in the fault during a trip over arc energy during a trip with the specified trip function; and    restoring the specified trip function following maintenance.    
     
     
         2 . The method of  claim 1 , wherein the specified trip function comprises a specified pickup current and a specified time delay and overriding the specified trip function comprises reducing at least one of the specified pickup current and the specified time delay.  
     
     
         3 . The method of  claim 1 , wherein overriding the specified trip function comprises reducing both the specified pickup current and the specified time delay.  
     
     
         4 . The method of  claim 1 , wherein the specified trip function comprises implementing one or more of an instantaneous trip function, a short delay trip function and a ground fault trip function and overriding the specified trip function comprises overriding each trip function implemented.  
     
     
         5 . The method of  claim 4 , wherein the short delay trip function is implemented with a specified short delay pickup current and a specified short delay time delay and wherein overriding the specified trip function comprises eliminating the specified short delay time delay.  
     
     
         6 . The method of  claim 4 , wherein the short delay trip function is implemented with a specified short delay pickup current and a specified short delay time delay and overriding the specified trip function comprises reducing the specified short delay pickup current.  
     
     
         7 . The method of  claim 6 , wherein overriding the specified trip function further includes reducing the specified short delay time delay.  
     
     
         8 . The method of  claim 4 , wherein the instantaneous trip function is implemented with a specified instantaneous pickup current and overriding the specified trip function comprises reducing the specified instantaneous pickup current.  
     
     
         9 . The method of  claim 4 , wherein the ground fault trip function is implemented with a specified ground fault pickup current and overriding the specified trip function comprises reducing the specified ground fault pickup current.  
     
     
         10 . The method of  claim 9 , wherein the ground fault trip function has a ground fault time delay and overriding the specified trip function further includes reducing the ground fault time delay.  
     
     
         11 . The method of  claim 1 , wherein overriding the specified trip function with a maintenance trip function comprises selecting one of a first maintenance trip function that results in a first level of arc energy in the fault during a trip that is less than the arc energy resulting from the specified trip function, and a second maintenance trip function that results in a second level of arc energy in the fault that is more than the first level of arc energy but less than the arc energy resulting from the specified trip function.  
     
     
         12 . The method of  claim 1 , wherein the low voltage circuit is multiphase and the maintenance trip function also causes a trip in response to a current imbalance in the multiple phases.  
     
     
         13 . The method of  claim 1 , wherein the maintenance trip function also causes a trip in response to a current reversal in the low voltage circuit.  
     
     
         14 . The method of  claim 1 , wherein the specified trip function incorporates a jumpered zone interlock providing a specified delay and overriding the specified trip function comprises eliminating the specified delay.  
     
     
         15 . The method of  claim 1 , wherein the specified trip function is overridden by substituting the maintenance trip function for the specified trip function, which is retained for restoring following maintenance.  
     
     
         16 . A low voltage circuit breaker protecting from arc flash resulting from faults in a protected low voltage power circuit comprising; 
 separable contacts;    current sensors sensing current in the protected low voltage power circuit;    a trip unit responsive to the current sensors tripping open the separable contacts in response to a specified trip function; and    maintenance means overriding the specified trip function with a maintenance trip function that results in reduced arc energy in the fault during a trip over arc energy during a trip with the specified trip function.    
     
     
         17 . The circuit breaker of  claim 16 , wherein the maintenance means comprises a maintenance switch operative between a normal position selecting the specified trip function, and a maintenance position selecting the maintenance trip function.  
     
     
         18 . The circuit breaker of  claim 17 , wherein the maintenance trip function comprises a first maintenance trip function that results in a first level of arc energy in the fault during a trip that is less than the arc energy resulting from a trip with the specified trip function, and a second maintenance trip function that results in a second level of arc energy in the fault that is more than the first level of arc energy but less than the arc energy resulting from the specified trip function, and wherein the maintenance switch has a first maintenance position for selecting the first maintenance trip function and a second maintenance position for selecting the second maintenance trip function.  
     
     
         19 . The circuit breaker of  claim 16 , wherein the trip unit comprises a zone interlock and the specified trip function is a short delay trip function with a short delay and the maintenance means open circuits the zone interlock and eliminates the short delay of the short delay trip function.  
     
     
         20 . A low voltage circuit breaker protecting from arc flash resulting from faults in a protected low voltage power circuit, said low voltage circuit breaker comprising: 
 separable contacts;    current sensors structured to sense current in the protected low voltage power circuit;    a trip unit responsive to the current sensors, said trip unit structured to trip open the separable contacts in response to a specified trip function; and    a maintenance mechanism structured to override the specified trip function with a maintenance trip function that results in reduced arc energy in the fault during a trip over arc energy during a trip with the specified trip function.    
     
     
         21 . The circuit breaker of  claim 20 , wherein the specified trip function of said trip unit is structured to provide at least one of an instantaneous trip function and a ground fault trip function; and wherein said maintenance mechanism is further structured to override each of said at least one of an instantaneous trip function and a ground fault trip function.  
     
     
         22 . The circuit breaker of  claim 21 , wherein the maintenance mechanism comprises a maintenance switch operative between a normal position selecting the specified trip function, and a maintenance position selecting the maintenance trip function.