Low Inductance Arrester Leads

An Alternative to Shorter Lead Lengths

Lead length can sometimes be an issue with arresters in that they inherently possess inductance. This inductance can cause a problem by developing a relatively high voltage along its length during a fast rising surge through the lead. If the arrester lead is in parallel with the protected insulation, this high lead voltage is also experienced by the protected insulation. Since the whole purpose of the arrester is to reduce the stress on insulation, it is counterproductive to have long leads that result in even more undesirable stress than needed. See Arrester Facts 001 on Arrester Lead length for much more on this subject.

Until a few days ago, I had not thought it possible to have a low inductance lead, and that we just needed to live with the inherent inductance of arrester leads and keep the lengths as short as possible. However while running tests this past week using a Marx generator, the shape of the impulse was modified using inductance free resistors. Now I have been using this type of resistor for 30 years and never thought that the method used to achieve low inductance resistors could also be used to achieve low inductance arrester leads. A quick Google search showed me that low inductance cable is used in other disciplines, such as the audio world. As it turns out, for high frequency music to sound good on speakers located many feet from the source amplifier, they require low inductance cable. If it can be done for such mundane tasks as making music sound better, then why not create low inductance arrester leads to improve surge protection. And yes, the method of achieving this low inductance cable for audio applications is exactly the same as it is for achieving low inductance resistors for surge generators.

The method is known as bifilar winding. The physics behind it is simple. If two inductors are wound in opposite direction and placed in the immediate vicinity of each other, they will cancel out each others electro-magnetic field. A bifilar wound resistor is created by winding resistive wire on a core using two conductors in parallel and winding them in opposite direction. An example of this method is shown for audio cables at this site

In this figure, two sets of counter-clockwise wound wires are shown.

  In this figure, the two pairs of counter-clockwise wires are wound together in a clockwise direction thus creating a non-inductive cable.

Typically arrester leads are considered to be about .3uH/ft to .4uH/ft. Using a winding method as shown above a .05uH/ft can be achieved.