The role of the maintenance bypass has been around since Uninterruptible Power systems were first introduced. The need for this package became obvious when a complete shutdown of the UPS was required, but the user could not tolerate an interruption (down time) of the load. The solution was to make available a second discrete power feed that the load could be switched to without interruption. This allowed for total isolation of the load from the UPS, freeing the UPS System for complete service or maintenance. This remains the fundamental role of the Maintenance Bypass System

The only stumbling block to adding this enhancement to the power design was the safety aspect both to the operator and the system. The first inclination was, and still is, to simply install two breakers in the critical switchboard, one to isolate the UPS from the load and a second to supply the load with utility power. This is precisely what the maintenance bypass system does, but the hazards are numerous if there are no controls in place to prevent the improper operation of these two very critical breakers.

The hazard that is most often recognized initially is the accidental closing of the utility breaker (MBP breaker) while the UPS is still online. The results of this act on the UPS can be quite disastrous and also includes the potential for an interruption of the power load as well. If there is no power available at the time from the utility, this scenario now poses a potential hazard to the Utility (Electrical Authority) in the form of a lethal back-feed to the utility from the UPS.

A second concern that the Electrical Authority commonly raises, is that even if operated in the proper sequence with the UPS on internal bypass, there still exists the potential to back-feed the lines. Especially if the two breakers are allowed to remain closed for an extended period. The potential for continuous reverse feed presents a serious electrical hazard to the Utility's electrical service personnel. A second concern raised by the Electrical Authority is that in many instances the breaker coordination is affected, jeopardizing the protection of the critical feeder due to two breaker paths feeding one line. In some instances leaving both breakers closed will feed twice the rated current to the cables.

These concerns help illustrate why Utility inspectors became increasingly less tolerant of maintenance bypass systems while at the same time consumers became more demanding for the use of them. It became such an issue with the Utility that at one time some inspectors would not pass a UPS installation with a maintenance bypass system unless it was mechanically interlocked ("break before make"). This met the Utility's concerns but negates the true purpose of the system. This scenario led to the development of the Maintenance Bypass System supplied by Critical Distribution.

This package is a self contained Maintenance Bypass System that is built with CSA and Hydro approval using an electrical interlock that works on a two wire fail-safe circuit. This "single pair" of wires require a dry contact closure within the UPS System ("UPS on Bypass") for the transfer enabled condition. Without this contact closed, the system prevents the closing of the alternate breaker. This allows the Maintenance Bypass System to address all of the above concerns.
 

The contactor version of our MBP System uses the same fail-safe inter-lock but utilizes contactors instead of breakers. The switching is similar to the motor enhanced version, in that the transfer can be executed at a single control switch.

An added feature of the contactor version is that in the event that the UPS output is lost and the utility source to the MBP is available, the MBP System will automatically default to the utility.
 

The Maintenance Bypass Package offer many advantages and it is worth noting that if ordered and installed in advance of the UPS installation, the commissioning and swing-over becomes a much simpler proposition. With advanced installation, only one scheduled interruption is required for the initial critical bus tie-in which can be scheduled for an opportune time.

The MBP System is available in a number of configurations from 50 amp to 1000 amp capacity, and for single or three phase systems from 120 to 600 volts. For system requirements of 60 amp or less, there is also a rotary switch version available. It does not employ the fail-safe features of the others but is more economical, which can be a factor in the lower power requirement end of the smaller capacity market. It should be noted that even though it is not a complete package, it is better than no maintenance bypass at all.

As a final note, if the user installs a UPS system without a maintenance bypass, there is no economic gain in the long run. No matter how dependable the UPS system is, it should still be maintained as effectively as possible. Without a maintenance bypass, many critical aspects of the UPS can not be maintained or serviced without critical bus downtime, and this is something the user is less and less prepared to accommodate. The Maintenance Bypass System also provides the customer with a "disaster recovery" path. In the event that the UPS System experiences a failure that prevents it from supplying any power to the critical bus (load shut down), the system can be open transition switched to return power to the load. This permits the user to re-power their systems even with the UPS path totally out of service.

There are many styles of this system in operation including versions that permit the service of emergency ATS packages. These systems are available in both uninterrupted and interrupted style with the cost of the interrupted version being considerably lower. Information and support for system application is available at your request. Please feel free to contact us with any questions or applications you may have.