Specifying your UPS

UPS modules are no different to other high impedance sources such as generators, they need special consideration that some or most professional design software don't do accurately. If you're an electrical engineer you should have the ability to make suitable decisions based on experience, qualifications and information.

Do your homework, double check any digital calculations and don't expect a lot of help from manufacturers. I have tried and they don't want to assist or can't. They supply a black box, however you are responsible for all infrastructure design and discrimination. If you purchase a UPS insist on a Factory Acceptance Test, you may have to pay extra so cost for it early on. Specify that they prove suitable disconnection in the factory under similar conditions that you have on site, certainly concerning protective devices and cable lengths. It will at least give you the confidence in your calculations. Ask for the fault tests to be recorded with a power analyzer and include in the O&M folder for the system.

In practice, when I have carried out this test at the factory, it was noted that the actual tests are slightly better than expected. I suspect this is the result of the instantaneous fault current for 1ms around 40x expected, before the inverter overload protection circuit cuts in, which may have started the protective device operating. As previously mentioned, attention needs to be paid to the ability to disconnect protective devices. Fixed equipment within 5 seconds and hand held equipment and socket outlets, special locations etc maybe at 0.4/0.2 seconds etc. Residual current devices, certainly on final circuits.

Again do your homework, find out what sort of earth leakage you can expect and use appropriate devices (500ma, 300ma, 100ma) are an option but IT and telecom loads produce earth leakage as part of normal service so this may not be an option. Time delay devices can help with discrimination. Removal and redesign of sub-distribution and final circuits to remove all large protective devices is good practice and helps with fault disconnection, but again this might not be an option.


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