The automatic transfer switch (ATS) is a critical system component of the emergency power supply system (EPSS), and proper maintenance of an ATS depends on the type of switch and its position in the critical power infrastructure. Imagine losing power during a natural or human-made disaster due to incorrect sensing or time-delay function. Or, the result of a maladjusted ATS control panel. A large percentage of automatic transfer switch failures are attributed to preventive maintenance.
Below is the sole reference in NFPA 110 (2019 Edition) referencing transfer switch maintenance.
NFPA 110, 8.3.4* Transfer switches shall be subjected to a maintenance and testing program that includes all of the following operations:
- Checking of connections
- Inspection or testing for evidence of overheating and excessive contact erosion
- Removal of dust and dirt
- Replacement of contacts when required.
NFPA 110, 8.3.5* Paralleling gear shall be subject to inspection, testing, and maintenance program that includes all of the following operations:
- Checking connections
- Inspecting or testing for evidence of overheating and excessive contact erosion
- Removing dust and dirt
- Replacing contacts when required
- Verifying that the system controls will operate as intended
The transfer switches and parallel gear are the brains of the EPSS; and while no frequency interval for maintenance is specified, it is strongly recommended that scheduled comprehensive reviews are performed by a ‘qualified’ technician quarterly with ‘internal’ maintenance performed once a year.
How an Automatic Transfer Switch works
Upon sensing the loss of normal power, the ATS logic signals the emergency power supply system (EPSS) to start. When the emergency source of power is available, and critical parameters such as voltage and frequency are within proper range, the ATS and/or paralleling switchgear will transfer to emergency power.
These steps are reversed upon the return of the normal power source. The ATS/switchgear logic will again verify the normal power is within the desired parameters and stable before retransfer. Once the ATS/switchgear returns to the normal position, the ATS/switchgear logic dictates a cool down period for the EPSS generator(s).
Four Automatic Transfer Switch types
- Break-before-make (open transition): The load will be interrupted during the transition from the normal to the emergency source.
- Make-before-break (closed transition): Allows transfer between two live sources of power without the loss of critical load.
- Delayed Transition (center off): Allows magnetic fields to collapse before reconnection to the alternate source.
- Soft Load/Unload: A form of closed transition that allows building load to be gradually transferred between two live sources of power.
Proper maintenance of all components of the emergency power supply system (EPSS) is essential and directly linked to the integrity of the critical power system. As it relates to NFPA 99 and 110; this system is only called upon to function in an emergency. Of course, this is the worst time for a malfunction of any one element. The total aspect of maintenance extends far beyond the routine tasks recommended to properly maintain any single element. Maintenance programs must be carefully reviewed, and maintenance provider selection is just as important as any other aspect of the EPSS.
Below are examples of the recommended annual maintenance tasks to be performed by a qualified OEM representative or certified technician:
- Part of any complete modern maintenance plan is infrared scanning / thermal imaging.*
- De-energize the switchgear (ATS’s equipped with an isolation bypass feature do not need to be de-energized).
- Remove the arc chutes and pole covers. Consult the manufacturer’s information for proper procedure. This step will allow visual inspection of the main and arcing contacts.
- Test and recalibrate all AC sensing and time-delay functions in the switchgear. Depending on the manufacturer, the steps required will vary.
- Vacuum the accumulated dust from the switchgear and accessory panels. Never use air to blow out dirt. Subjecting the transfer switch mechanism to compressed air may have a detrimental effect by forcing dirt and debris into the switch mechanism.
- Inspect for moisture or signs of previous wetness or dripping.
- Clean grime with an approved non-conductive solvent. Consult the OEM for a recommendation.
- Inspect all main arcing contacts for excessive erosion and/or pitting. Arcing contacts are intended to be sacrificial by nature. They take the brunt of the energy when making or breaking the load. Careful attention should be paid to these contacts.
- Inspect all control relay contacts for excessive erosion and discoloration due to excessive heat.
- Manually operate the main transfer movement to check proper contact alignment, deflection, and wiping action.
- Check all cable and control wire connections to the transfer switch controller, sensing panels and other system components. Tighten if necessary.
- Re-energize the switchgear and conduct a test by simulating a normal source failure.
Although the tasks listed above do not appear especially extraordinary; these steps require specific training and tooling. NFPA 99 and 110 specifically state maintenance and testing activities will be performed by qualified/competent personnel.
NFPA 110, 8.4.8 EPSS components shall be maintained and tested by qualified person(s).
NFPA 99, 18.104.22.168.1.4(C) Test Personnel. The scheduled tests shall be conducted by competent personnel to keep the machines ready to function and, in addition, serve to detect causes of malfunction and to train personnel in operating procedures.
The total aspect of maintenance extends far beyond the routine operational testing performed by facility engineers. The example of the annual ATS maintenance illustrates the requirement of these procedures to be carried out by a dedicated, trained professional.
A surface comparison of maintenance proposals by competing vendors may be misleading. An aggressive maintenance company may state they can do everything that is ‘required’. Go beyond that…ask the tough questions…and ask for references.
Special thanks to Steve Works with Blanchard Machinery for assistance in editing this article.