Charlie_in_TX
Member
- Location
- Cypress
We have all seen the standard circuit. Starting from the X1 side of the transformer connected to the fuse connected to 'START'/'STOP' circuit connected to the contactor coil connected to the overload protection finally returning to the X2 grounded side of the transformer. The 'START' button has a aux contact in parallel to latch the start signal. A safety benefit is that when power is removed from the contactor coil for any reason, upon the power returning to normal, the start sequence must be re-initiated.
Standard stuff. I had it explained to me as controls on the hot side, load in the middle and safety permissives on the right grounded side. The permissives are always normally closed.
But why? Is it in the NEC someplace? Is it some other code?
I am working on an industrial automation system. I work for the controls contractor. The electrical contractor is building the MCC. They are attempting to change the above standard.
Design issues (but are they code violations?)
1. 'HAND''OFF''AUTO' rotary switch. In this system 'HAND' should be labeled 'ON'. It applies power to the contactor coil until it is turned 'OFF'. The problem? If there is a motor shutdown (because overload heaters, emergency shutdown device (ESD), etc.), when the the condition is corrected, the motor will start immediately without a new 'START' command.
2. Reverse logic in the ESD. Let me explain. They are using the NO contact in the ESD to power a relay. The ESD is at the motor, the relay is 150ft away in the MCC. A NC contact in the relay is used in the HOT side of the control circuit. Problems. A. The permissive is in the HOT side of the load not the grounded, 'neutral' side. B. The permissive is not 'fail safe'. It is possible for the motor to run without the cable to the ESD connected.
Standard stuff. I had it explained to me as controls on the hot side, load in the middle and safety permissives on the right grounded side. The permissives are always normally closed.
But why? Is it in the NEC someplace? Is it some other code?
I am working on an industrial automation system. I work for the controls contractor. The electrical contractor is building the MCC. They are attempting to change the above standard.
Design issues (but are they code violations?)
1. 'HAND''OFF''AUTO' rotary switch. In this system 'HAND' should be labeled 'ON'. It applies power to the contactor coil until it is turned 'OFF'. The problem? If there is a motor shutdown (because overload heaters, emergency shutdown device (ESD), etc.), when the the condition is corrected, the motor will start immediately without a new 'START' command.
2. Reverse logic in the ESD. Let me explain. They are using the NO contact in the ESD to power a relay. The ESD is at the motor, the relay is 150ft away in the MCC. A NC contact in the relay is used in the HOT side of the control circuit. Problems. A. The permissive is in the HOT side of the load not the grounded, 'neutral' side. B. The permissive is not 'fail safe'. It is possible for the motor to run without the cable to the ESD connected.