With the advent of microprocessor based regulator controls, the Hi & Low override function became a feature easily added to the design of the control. In fact, in a menu based control, no additional hardware is even needed! In the age of analog designs, adding this feature meant adding an additional module to the control cabinet. Most manufacturers had one available. Beckwith called it a “backup relay”, GE called it a “First House Protector”…which gives us a hint as to its function.
The problem with the additional modules is they were a common failure point. The motor run voltages for raise and lower were run through normally closed relays that never opened unless there was a voltage excursion beyond the set-point for the backup. Relays have “wiping action” that cleans the contact of contamination every operation. Never operating? Never cleaned. The relays would open and cause a control malfunction just sitting there.
This is not a problem for logic based protection. It uses the main output relays just like a normal raise or lower operation, and these get plenty of exercise in most cases!
Then, why do we have this backup (or “First House Protection”)? The main reason is to protect customers immediately downline from Line Drop Compensator settings that are wrong due to changes in load characteristic or errors in calculation to begin with. This logic based backup WILL NOT protect the consumers on the load side if there is a hard failure in the control that causes runaway. The failure in the control is the same hardware for voltage control and override. If one fails, they both fail!
We see controls come in that customer says will not maintain voltage, and many times we see an upper limit programmed into the override that is within the normal deadband of the control settings! We change settings, do a 24 hour test and charge for bench time.
If you are not using Line Drop Compensation, we recommend not using Hi/Low Voltage overrides, unless your primary is so soft that it can’t stay in regulation itself. If you use Line Drop Compensation, using override protection is a must, just make sure the numbers entered are outside the edges of the deadband.
So how can does one protect against a hardware failure run-away? An external backup is the only way. Just make sure that you exercise it fully when doing routine regulator checks in the field to help keep the relay contacts clean.
Work safe out there!