Normally the Insulation Resistance value should be around 1 M ohm. However this problem is usually caused by the way this test is carried out. The usual method for carrying out by such a test is shorting out phase and neutral and measuring them against earth. However when doing so you are connecting all the internal resistors in parallel which causes a low reading of resistance. If the test is done using a high voltage, 500V DC, the unit will also allow excess Voltage to flow to Earth (Ground) to protect the load but this will show as a drop in resistance for measurement applications. Most European Regulations e.g. IEE 16. edition, section 10.3.3, require all electronic components to be disconnected before carrying out such tests. The surge protection in our UPS’s is achieved by using MOV’s which have a certain let through Voltage.
Please note that using 500V DC to carry out this test, may damage the UPS and such damage will not be covered by APC warranty. In our factory we carry out such a test using 2000V but with the critical electronics disconnected from earth. This is further explained in the attached document.