How to Perform Generator Testing? [Before It Ends up Testing You]

The likelihood of experiencing a power failure in the UK is, believe it or not, relatively high. In 2017-2018 across the six Distribution Network Operators (DNO’s) there was an average of 44 interruptions per 100 customers, according to the RIIO Electricity Distribution annual report 2017-18.

So, at some point, it is safe to say that if your generator and emergency power system hasn’t been tested, a network fault, an extreme weather event or some other external factor will cause an interruption to your sites electricity supply and test it for you.

During my time at Vertiv and its various incarnations, I have lost count of the number of emergency call-outs that we have received stating “Site has experienced a mains failure and the generator has failed to kick in” or “Site has had a power cut, generator has started but changeover system has failed to operate”. When these faults have subsequently been investigated, most of these failures would have been identified and subsequently rectified had the client followed a robust testing regime.

Why Test Your Generator?


Simple - to prove the emergency power system and discover any faults in a controlled environment.

Regular testing will also give you the confidence that your generator and emergency power system will have the resilience and capacity to keep the lights on and your business running, irrespective of any external event that may occur that has the potential to disrupt your sites electricity supply.

There is a marked reduction in the amount of emergency calls we receive from our customers who have adopted a robust generator testing regime as opposed to those who don’t. It goes without saying, the customers with a thorough testing regime also have full confidence in their system being able to perform when required.

The Most Common Emergency Power System Faults You Can Prevent by Testing It

Faults that can be identified during testing can be as simple as a failed or failing starting battery on a generator to a more complex control failure or even to discovering issues with the initial design of the system, such as incorrect earthing arrangements or fuel transfer system. You might also wish to learn more about fuel polishing and how can clean diesel fuel help you prevent generator failure.

I have also come across several customers who carry out regular generator testing but were not carrying out the test in the correct way, for instance, by only using a test switch or removal of a sensing fuse. This would only simulate a mains outage that could potentially mask issues such as failed or failing switchgear tripping batteries.

Another thing that is sometimes overlooked are the under/over voltage and frequency settings in the UPS and Changeover systems and that they are compatible with each other, for example:

If the UPS undervolts setting is set at 225v L-N then the UPS will go onto battery at this point to support the load. If, however, the generator changeover undervolts relay is set at 220V L-N, then the generator will not register a mains failure and therefore not be called for. This will mean that the UPS would carry on supporting the load, on battery until the mains returns above 225v, the mains drops low enough for the generator to kick in or the UPS batteries totally discharge resulting the load being lost.

Do you know if your UPS and generators are correctly set up? Don’t leave it to faith. Reach out and let’s test your generators, before they test you.

On some systems that employ Air Circuit Breakers (ACB’s) as a means of changeover switchgear, I have seen issues where Undervoltage coils are being used as a means of automatically opening the switch in the event of a mains failure.

If, however, the mains failure was merely a ‘blip’ it would still cause the switch to trip open but as the outage hadn’t been long enough to complete the changeover process the mains ACB could potentially become “locked up” in the open position resulting with the site not being supported by either mains or generator power.

Introducing a wide range of scenarios into a good testing regime would highlight issues such as this and allow for any remedial work to be carried out to remedy any faults found in a controlled manner.

Load bank testing where an external dummy load is connected to the generator can also be extremely valuable in discovering problems with the generator such as airflow problems or fuel supply problems that may not be apparent during off load or low load tests. They can also be invaluable for assisting with fault finding where an issue only occurs when the generator is under load. Vertiv has its own fleet of various sized load modules that can be delivered to your site, anywhere in the UK.


Types of Generator Testing

  1. Off Load Tests

During this type of testing, the generator is started and run manually with no load switching operation involved. What happens during these operations:

  • The basic operation of the generator is being tested.
  • Off load running should be kept to a maximum of 10 mins as prolonged offload operation can cause long term damage to the engine.
  1. Load Bank Testing
    • It can be closely controlled.
    • The standard duration is 3 hours, but it can be tailored to any duration dependant on the installation.
    • It can be used as a tool to aid fault diagnosis.
    • It causes minimal disruption to site.
    • It’s relatively cheap to carry out.
    • Load bank testing can be resistive only (most common) or resistive/reactive (normally used for commissioning and testing where generators synchronise together). A resistive/reactive load test is typically 25% more than the cost of a resistive only test, due to the specialised nature of the load banks required.

      Load bank testing is conducted when the generator is under full rated load & overload condition (if available) and tests the thermodynamics of the installation as well as fuel flow and engine governor set up. The main characteristics of this operation are:

    1. Building Load Testing/Mains Fail Testing/Black Building Test

This operation tests the full system, including downstream switchgear and changeover controls. The defining traits of a black building test are:

  • It’s normally done out of hours, but best to be done as close to real life operation as possible so a true reflection of what would happen in a true outage can be replicated.
  • It’s a “Real life” test if conducted properly.
  • It is usually recommended that site switchgear is properly maintained before embarking on this type of test.



  1. Integrated System Testing (IST)

The integrated system testing is conducted on the whole emergency power system in a building using lots of different scenarios, including simulated failures of various parts of the system to prove resilience.




We recommend a combination of all the above generator tests, carried out at different times over the course of a year to form a robust and thorough testing programme that would ensure all aspects of a buildings emergency power system are regularly tested and operating as they should and within parameters.

Moreover, we’re here to support you with all types of testing and we can work with you to devise the correct testing regime that is most suitable for your situation. We can provide highly trained engineers familiar with generator, switchgear and UPS systems to assist you on site with any of your testing requirements, any time of the day or night 365 days a year.

Our experts can also help you with the complete maintenance and repair of your UPS, generator and switchgear systems.

Don’t wait until your generator test you, contact us so you can test it first.


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