Conductor Length Issues
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. When lightning current flows in a conductor, the inherent inductance of the conductor develops a large voltage, which appears between the two connection points. If this voltage is in excess of the insulation or coating strength, arcing will occur.
A suggested guideline for lead length, due to these factors, is a total of 12" (300mm) including both conductors. This may not be possible in some cases, but the length should still be kept as short as possible.
The actual test data shown below is measured across a specific DEI device during a 50,000A lightning current event.
|Lead Length||Voltage Developed||Comments|
|Zero||100V||Voltage across device only|
|12" (300mm)||1500V||Total of 12" of lead|
Note that for even short lead lengths, the voltage across the connection points increases very rapidly. Most typical field installations use much more lead length than 12" and also use a protective device such as a spark gap arrester, which has a much higher conduction voltage. This combination effect increases the voltage across the insulated joint (or other structure) to a level that may cause arcing and insulation breakdown.
This situation can be addressed by simply locating the protective device very close to the object being protected, and choosing a protective device with the lowest possible conduction threshold.
Ideally, a DEI product could be mounted directly on an insulated joint, providing both the physical and electrical connections necessary, and minimizing lead length. Various mounting kits and formed bus bars are available through DEI to assist with this task.
For a more complete discussion of inductance effects, see the article "Lead Length Effect on Over-Voltage Protection Due to Lightning Surge Current."