Insulated Joint Protection

A common application of DEI products in cathodically protected systems is to provide over-voltage protection due to lightning. A typical example would be that of providing over-voltage protection of an insulated joint in a gas transmission pipeline. 

Hazardous (Classified) Locations are defined in the US National Electric Code (NEC), Articles 500-505, and in other international standards such as the IEC, and describe the requirements of electrical products used in these locations. Decisions on the classification of a particular site depend on several factors, including:

Over-voltage protection is greatly affected by the proximity of the device relative to an insulated joint or other structure being protected. This effect is independent of the protective device being used, as it is mainly due to the length of the conductor. 

DEI 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 DEI product rating. This measurement should be made between the same points to which the DEI 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.

Many user sites are formally classified as hazardous locations, according to various standards. Other sites may commonly have explosive vapors or gases present, but haven't been identified as a "hazardous location" due to unfamiliarity with the classification requirements.