With these USB combined sockets becoming more and more popular it’s likely that when you come to install or test within a domestic property you’ll stumble across one. Statistics show that every house hold has multiple USB devices making these sockets perfectly placed at market.
USB devices are normally powered by 5v dc, this means that the USB sockets must convert the nominal 230v ac to 5v dc. This is achieved using a switch mode power supply (SMPS). Most sockets available today have this connected directly across the un-switched side of the Line-Neutral circuit.
This arrangement is problematic when it comes to insulation resistance testing because just like a neon or a missed lamp the SMPS will place a resistance of approximately 200 kΩ thus failing an insulation resistance test. Additionally and certainly worst this arrangement will be susceptible to damage from a 500v insulation resistance test.
Correct procedure for testing USB combined sockets
The purpose of the insulation resistance test is to verify that the insulation of conductors provides adequate electrical insulation and is not damaged and that the live conductors and protective conductors are not short circuited.
BS7671 requires that insulation resistance is measured between all live conductors and between live conductors and the protective conductors.
Guidance note 3 states that “For circuits and or equipment vulnerable to the applied test voltage, the test is made with the Line and Neutral conductors connected together and earth.”
So applying the advice from guidance note 3, I would simply test between Line/Neutral combined and earth. This would of course satisfy the requirements laid down by the regulations however it would NOT show a potential fault between line and neutral. Therefore I would perform a second test, a soft test at 250v (dc) between line and neutral. I already know that this test is likely to fail to meet the >1MΩ minimum value so I will use interpretation to make a decision based on the fact that I’m expecting to see a value of 200 kΩ as stated above, therefore any result lower than this will require further investigation.