3. What insulation resistance testing does and how it is performed
The theory behind IR testing is derived from Ohm’s Law. Apply a known voltage between conductors and earth then measure the leakage current drawn. From this result a figure for IR can be calculated with the result expressed in Ohms.
Testing of IR integrity can take two forms, and both have relative merits.
Installing a Line Insulation Monitor (LIM) allows continuous surveillance of the system while in service. Automatic alarms can be generated if the IR parameters exceed pre-defined limits, especially useful when integrated with a SCADA system. LIMs typically apply a low-frequency AC voltage.
Periodic or on-demand testing can also be carried out on a de-energised system using an IR tester that applies a high DC voltage for a set period of time. The results can be interpreted to identify the approximate nature as well as the presence of a fault.
4. Mandatory insulation resistance testing may cause copper loss, but it can be mitigated
IEC 60364 covers the insulation monitoring requirements of IT (Isolation Terra, unearthed) systems such as those used on subsea equipment. However, where an insulation fault allows seawater ingress, it is known that LIM devices can exacerbate damage to the cable by promoting an electrochemical reaction that degrades the copper itself.
The V-LIM system from Viper Innovations can help to mitigate against this by regulating the applied voltage used during operation, minimising the copper loss while continuing to accurately monitor the IR. When compared to alternate systems lacking this capability, the time available to rectify the issue before eventual failure is greatly increased.