The case for load testing standby generators

An expert explains how to ensure the reliability of back-up power before outages occur
Monday, September 14, 2015
By Bhavesh Patel

How reliable is a facility’s on-site power system? Waiting until an outage occurs, possibly leaving the facility in the dark, is not the time to find out. The better way is to periodically test the standby generator under load to help ensure it is ready to run when needed.

Facility managers may be reluctant to test the standby generators, transfer switch, and other components under load because of cost, inconvenience, or other reasons. If the testing reveals a problem, then what? However, there are several very important reasons to test the onsite power system.

Why test

A standby generator that is idle most of the time needs to be exercised to prevent the system from breaking down due to lack of use. Testing helps facility managers proactively diagnose and fix problems and determine if an aging standby system is due for replacement or component upgrades — at a time the system is not needed.

Testing a generator with a load bank eliminates the effects of wet stacking, which is the build-up of carbon deposits in the exhaust. This build-up can adversely impact generator components including piston rings, turbochargers, injector nozzles, the combustion chamber and the exhaust pipe, reducing engine life. Running the generator at the required percentage of the rated load burns off carbon deposits in the engine.

The technicians conducting the load testing will also monitor the engine for leaks, temperature and oil pressure and, based on their observations and readings, be ready to disconnect the load if problems arise.

Ultimately, the cost of regularly scheduled maintenance and testing is low compared to the financial fallout of even a brief outage, and its negative impact on productivity and, if customers are affected, on reputation. Plus, if a facility fails to test and keep records, and the standby systems fails after a utility power outage, an insurance provider could refuse to protect the building owner against claims from clients for loss of business.

How to test

The most efficient and safest way to evaluate the health of a standby power system is to schedule periodic field testing of the standby generator, the transfer switch, the paralleling switch gear and all the associated components under the equivalent of a live load. The test should be conducted by qualified technicians under controlled conditions.

(Turning on a generator without a load attached is like turning on the ignition in a car left up on jacks in a garage for a long time and leaving the car in park. There is no way to know if the vehicle will operate on the road. Just hearing the motor run doesn’t mean the car can be driven or the tires are aligned.)

Load testing is typically done by applying an outside electrical load (usually a load bank) to the standby generator. The load bank, which is generally portable and transported to the facility on a flatbed trailer or the back of a truck, is equal to the total load the generator would be expected to carry if utility power became unavailable. This type of test does not disturb the live load and eliminates the risk of causing a loss of power should the system not function as intended.

Once the load bank is connected to the standby generator, the generator is turned on and — presuming the transfer switch operates as intended — operated for a predetermined period of time. The attending technicians make note of any operational problems, which the facility manager can then address at a non-critical time.

When to test

How often the testing should occur depends on the classification or use of the facility, local building code, the original equipment manufacturer’s recommendations for the application, and operating conditions (climate, weather, humidity, ambient temperatures, and exposure to dust, sand, and other airborne contaminants).

For instance, at data centres where business disruptions due to loss of power can be costly, exercising the standby generator may be a weekly practice, with load testing conducted yearly at a high level load and monthly at a lower level load.

(CSA 282 from the Canadian Standards Association calls for a monthly 60-minute building load test at minimum 30 per cent of the rated load for emergency generators, which could be considered a guideline for standby generators for on-site power systems.)

While some facility managers may balk at the cost of load bank testing, the expense is substantially less than the costs of an unexpected outage that disrupts business. Scheduling periodic field testing and proactively diagnosing and fixing problems when the standby generators isn’t needed will help ensure a facility isn’t left in the dark when the generator is needed.

Bhavesh S. Patel is vice president, Global Marketing for ASCO Power Technologies, a business of Emerson Network Power. He can be reached at or 800-800-ASCO.

2 thoughts on “The case for load testing standby generators

  1. Thanks for the above valuable information, but what about if there are no ready terminals to connect the load back?

    I think it will be very risky to disconnect the load cables from the generator and connect the load back direct to the generator, maybe during this test the utility will shut down and the generator will be required. Also in this case the ATS well nut be tested.

    What it’s the solution in this case?


  2. Facility needs to have some sort of quick disconnect installed as part of testing. If not, then there is a disruption to existing facility when testing is conducted.

    Those facilities that do not have provision…does testing during off-hours or weekends to minimize impact of disruption.

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