Conductor Length Effects Upon Over-Voltage Protection From Lightning Conditions

Dairyland products are commonly used in cathodically protected systems to provide over-voltage protection due to lightning. Dairyland decouplers and over-voltage protectors are excellent choices for this application, and they carry third party certifications for high levels of lightning current, ensuring their capabilities. However, an often-overlooked factor during installation of over-voltage protection products is the impact of conductor lengths used to connect the device to the protected structure. Paying close attention to the installation method can have a direct impact on the end result of providing adequate lightning protection.

Evaluating Conductor Inductance

Due to the inherent inductance of a conductor, when lightning flows through it, a large voltage may develop between the connection points:  The longer the conductor, the greater the inductance and the greater the voltage developed across the conductor. If this voltage exceeds the insulation or coating strength, arcing will occur. As a result, when any product is used to provide over-voltage protection from lightning (e.g. such as across an isolation joint) it is extremely important to recognize the detrimental effect of long conductors used to connect the protective device. To achieve maximum performance, it is critical to keep the conductors as short as possible. Consider the following formula:

V (total) = V (device) + V (conductors)

The conductor length is the total length of the two conductors 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 conductors themselves.

Of course, to achieve successful protection, the total voltage across the isolation joint must be below its insulation withstand value. Normal procedure would be to select a protective device with the lowest allowable clamping voltage for the application. Dairyland solid-state devices have a threshold voltage of typically several volts, while spark gap arresters fire at hundreds to low thousands of volts. However, any device also has a total voltage that appears across its terminals during the flow of lightning current, due to the same inductive effect described in this article – a value that is above its threshold voltage.

Example

As an example, assume that the device selected has a clamping voltage of 500 volts at the highest value of lightning surge current anticipated. Referencing the above formula, this results in:

V (Device) = 100 volts

Next, the effect of conductor length will be considered. Any conductor, 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 conductor inductance (L), and the rate of rise of the lightning surge current (di/dt) as shown in the following formula where L is in microhenries (µH), and (di/dt) is in amperes per microsecond (A/µsec):

V (Conductor) = (L) • (di/dt)

Based on measurements of the conductor sizes normally used to connect Dairyland products, a reasonable estimate for conductor inductance is 0.2 µH per foot of conductor 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

V (conductors) = 0.2 x 13,000
Result: 2600 volts per ft. of total conductor length

The total voltage that would appear between the two connection points of a surge protective device given the above,realistic, parameters is:

V = V (Device) + V (Conductors)
= 100 + 2600 = 2700 volts if total conductor length = 1 ft.
= 100 + 5200 = 5300 volts if total conductor length = 2 ft.
= 100 + 7800 = 7900 volts if total conductor length = 3 ft.
Note that conductors for each terminal of the device must be one-half of the length shown, assuming equal conductor lengths.

Based on this data, it is easy to recognize the predominant factor that determines the maximum voltage that will appear across the isolation joint (and often, other protected components) is determined by conductor length, not the protective product threshold voltage.

Keep the Conductors Short!

A suggested guideline for conductor 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 was measured across a Dairyland device during a simulated 50,000A lightning current event. Note that for even short conductor lengths, the voltage across the connection points is significant.

Most typical field installations use much more conductor length than 12″ and may 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 isolation joint (or other structure) to a level that may cause arcing and insulation breakdown.

Choose the Right Mounting Accessories!

To aid in minimizing over-voltage due to lightning, Dairyland provides numerous mounting accessories designed to minimize the effects of conductor inductance, which can be found in the accessories section of our website.

Where conventional insulated copper conductors are used, keep the conductors as short as possible, preferably less than 12” total, and as close together as possible to minimize conductor inductance.

If you should have questions about this or other products that Dairyland manufactures, please feel free to contact our technical support team.