Wiring Accessories
Testing requirements and USB socket-outlets
As a wholesaler, are you having to deal with the frequent returns to your store of USB socket-outlets that have been faulty following insulation resistance testing? If so, Jake Green, head of technical engagement with Scolmore Group can shed some light on the issue.
W
ith USB socket-outlets, the charging unit is built into the back of the socket and this is sometimes fixed to the on position. This means that the switches on the socket that control the 13 amp plug element have no connection to the USB charging point, with the result that is permanently connected. This causes problems when carrying out insulation resistance testing, as the electronic components of the USB charging unit are part of the circuit and therefore the 500V that electricians use to
test them could damage those components resulting in a faulty charging port. The Click range of USB sockets has been designed to withstand the 500V insulation test due to the nature of the circuitry built within the USB socket.
Chapter 64 of BS 7671 details the general requirements for initial verification, and Section 643 deals with the requirements for testing. Here we briefly consider the requirements for insulation resistance testing and the care that should be taken when testing after second fixing has occurred.
Insulation resistance testing Insulation resistance testing is carried out to confirm that there are no inadvertent connections between live conductors, and between live conductors and Earth before the installation is energised (643.3.1).
The insulation resistance test is a ‘dead’ test, and the normal supply should not be ‘live’. To this end the person carrying out the test should ensure that the circuits to be tested are isolated and the normal power cannot be energised inadvertently.
The minimum required insulation resistance values in megohms is detailed in Table 64, although a value significantly in excess of these
24 | electrical wholesalerMay 2023
ewnews.co.uk Figure 1: Testing live conductors to Earth
Insulation resistance testing is carried out to confirm that there are no inadvertent connections between live conductors, and between live conductors and Earth before the installation is energised (643.3.1).
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