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Lead Length
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Effect on Over-Voltage Protection Due to Lightning Surge Current
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. The ISP and PCR are excellent choices for this application, particularly when over-voltage protection is required both due to lightning and AC fault current. (AC fault currents are a factor anytime a pipeline is in or near the same corridor as a power utility line).
When any product is used to provide over-voltage protection due to lightning between two points (e.g. such as across an insulated joint) it is extremely important to recognize the detrimental effect regarding the length of the leads used to connect the protective device. The lead length is the total length of the two leads required
The total voltage developed between the two connection points of any device due to a lightning surge is the sum of: (1) the clamping voltage of the device selected and (2) the voltage developed by the leads themselves.
That is:
V = V (device) + V (leads)
Normal procedure would be to select a protective device with the lowest allowable clamping voltage for the application. For our insulated joint example it would be normal to select a device that might have several hundred volts across it due to lightning surge current; well below the insulation breakdown of a typical insulated joint. Therefore, for illustrative purposes, assume that the device selected has a clamping voltage of 500 volts at the highest value of lightning surge current anticipated.
This results in:
V (Device) = 500 volts.
Next, the effect of lead length will be considered. Any lead, subject to a current with a very high rate change, will develop an inductive voltage drop along its length. This voltage drop is a function of the lead inductance (L), and the rate of rise of the lightning surge current (di/dt).
Therefore:
V (Lead) = (L) (di/dt)
where L is in microhenrys (µH), and (di/dt) is in amperes per microsecond (A/µsec).
Based on measurements of the lead sizes normally used to connect DEI products, a reasonable estimate for lead inductance is 0.2 µH per foot of lead length. (Other sources have suggested a value as high as 0.4 µH / ft.) More recent field measurements of lightning surge currents (direct strokes) indicate that the rate-of-rise (i.e., di/dt) in half of the measurements was 13,000 amperes per microsecond, with the maximum value measured at 60,000 amperes per microsecond.
Next, some sample calculations will be shown using the above data and the following assumptions:
L = 0.2 µH /ft
di/dt = 13,000 A/ µsec
Then, V (leads) = 0.2 x 13,000 = 2600 volts per ft. of total lead length.
The total voltage that would appear between the two connection points of a surge protective device given the above, very realistic parameters, is:
V = V (Device) + V (Leads)
= 500 + 2600 = 3100 volts if total lead length = 1 ft.
= 500 + 5200 = 5700 volts if total lead length = 2 ft.
= 500 + 7800 = 8300 volts if total lead length = 3 ft.
Each lead must be one-half of the length shown, assuming equal lead lengths. Note that the predominant factor that determines the maximum voltage that will appear across the insulated joint (and often, other protected components) is determined by lead length. We highly suspect that in some cases where an over-voltage failure due to lightning occurs, it is not due to a problem with the protective device, but rather due to excessive lead length.
Based on this more recent lightning surge current data, DEI has revised its original recommendations and now recommends 3000 volts per foot of lead length as a reasonable value. We have not used the extreme values noted above because that data was for direct lightning strokes. Most of the equipment in cathodically protected systems will not likely be exposed to the full current value of a direct stroke.
To aid in minimizing over-voltage due to lightning, DEI is willing to provide custom fabricated copper bus for specific applications to connect products like the ISP or PCR so as to minimize the effects of lead inductance. Where conventional insulated copper leads are used, keep the leads as short as possible and as close together as possible to minimize lead inductance.
To our knowledge, there have not been any lightning caused failures to any equipment protected with DEI products. The above information is provided to minimize potential failures that could occur due to lack of knowledge of lead length effects.