Determining Maximum DC Voltage
Dairyland products have a set value where they transition from the DC blocking mode to the shorted mode, called the threshold, or blocking, voltage. The site’s maximum DC voltage and induced AC voltage, if any, should be known in order to select the proper Dairyland product rating. This measurement should be made between the same points to which the Dairyland device will be connected. For example, at an insulated joint, use a voltmeter to measure across the flange faces to determine the DC voltage difference. Likewise, at a pipeline in a station, measure between the pipeline and the grounding grid. Consider any stray DC voltage from rail or CP systems that will increase the needed product voltage threshold.
Key: The Dairyland product selected should have a DC blocking voltage that is greater than the measured DC site voltage for correct application.
A common question from users is whether induced AC voltage in excess of the published Dairyland blocking voltage rating will put the device into conduction. The answer to this question is “No” because a Dairyland product such as the PCR, PCRH, SSD, or ISP is a low impedance AC path at the same time that it blocks DC current. AC induced voltage is immediately collapsed below the device threshold, and AC steady-state current flows through the device, without an effect on the cathodic protection voltage, when the proper steady-state AC current rating is selected.
A special case is the Over-Voltage Protector (OVP and OVP2), which blocks both DC and AC voltage up to a predetermined threshold level. This device should not be applied to sites with induced AC voltage, as it may put the OVP into conduction under normal conditions. It may be applied to such a site, however, if another Dairyland device, such as the PCR, is being used to first mitigate the induced AC. The DC plus peak AC voltage must always remain below the OVP blocking voltage for proper application.
Related issue: AC Voltage Mitigation