Earth leakage current with UPS

Introduction

When electrical equipment is connected to people for example in a medical application extra regulations apply.

Such regulations are designed to ensure that a patient cannot be harmed by a fault in the electrical supply.

Specific details for patient connect equipment are outlined in IEC 601 / EN 50601

UPS measurements

Customers with UPS equipment may test the equipment using off the shelf testers to ensure it is safe in person-connected situations or where the general integrity of the equipment is being checked.

We have found instances where the test results are confusing because the test equipment performs an earth leakage measurement between Live and Neutral and also between Neutral and Live.

The reversal of the input connections can give rise to unexpected results.

Our UPS products are designed and manufactured to meet the EMC requirements of current legislation. In normal use the live and neutral supplies are connected the `right' way around. To meet the regulations the input filtering may not be symmetrical about the two input wires and earth.

For example a capacitor connected between live and earth will see about 230Vac and a similar sized capacitor fitted between neutral and earth will normally see less than 10Vac. If these are checked at 50Hz for leakage current the results will be different by more than an order of magnitude.

So an off the shelf tester measuring the combined effect of several such components will see different results for earth leakage depending on the polarity of the test.

Recent calculations on the input filter fitted to a 1kVA MicroBak showed expected figures of 41 and 759 micro amps. Actual measurements provided figures of 75 and 800 micro amps.