A typical uninterruptible power supply (UPS) requires EXTRA calculations to be performed before installation and sign-off.
In the illustrated case the UPS has no galvanic isolation and the earth + neutral go through the unit. The recommended bypass switch means that it is possible for the distribution wiring to see the low impedance mains supply.
For the first part of the calculations the UPS should be ignored and the wiring calculated out in the normal way. It is unlikely that when the UPS is introduced into the system that the rating of any wiring will have to be increased EXCEPT the input supply.
Most UPS equipment is not 100% efficient. In addition to supplying the full rated load current modified by the UPS efficiency there is a need to support the battery charger as well. The UPS manufacturer should provide worst case input power needs. For longer supply runs this often means that a larger wire is required than expected for an installation where there is no UPS.
For the first part of the testing the UPS should be isolated and the distribution connected to the main supply via the bypass switch. Normal checks should be applied using calibrated equipment.
NOTE: some modern pulse-based testing equipment can damage electronic equipment which should normally be disconnected during testing.
The second consideration is where the distributed power is supplied by the static inverter in the UPS. This has real and apparent impedance. The apparent impedance is electronically controlled and should be specified by the UPS supplier as a worst case current available from the UPS.
For user safety the installer must ensure that the earth loop impedance is sufficiently low so that it provides a voltage inside regulations for the worst case fault current available from the UPS. It should be appreciated that under these conditions over current fault protection may not clear.
For fire safety the installer needs to evaluate that all the distribution wiring will not overheat or it must clear protection under fault conditions. If the current is below the continuous current rating of the wiring the apparent source impedance may mean that the protection does not clear.
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