By Doug Criner

Manufacturers of implantable heart pacemakers caution that electromagnetic interference and magnetic fields may disrupt a pacemaker’s operation. For example, one pacemaker manufacturer, Medtronic, cautions its wearers to "check with your doctor before working with the following equipment: arc welders, electric furnaces, large magnets (such as those used in some stereo speakers), power plants, or transmission lines."  I doubt that most doctors would be able to answer the question.

Pacemakers are computers that can be disrupted by EMI. In addition, magnetic fields can put a pacemaker into a test mode—doctors can do this deliberately by placing a magnet on the patient’s chest.

What Are the Limits?

Unfortunately, pacemaker manufacturers do not specify a maximum field strength below which there is no concern to the wearer. Certainly, some models are more susceptible to such problems.

In the absence of specific criteria from the manufacturers, some experts have developed guidelines. For example, Oak Ridge National Laboratory has established the following limits to avoid (see www.ornl.gov.)

        ▪ 60-Hz magnetic flux densities above 0.1 mT (1 G)

▪ 60-Hz electric fields above 1 kV/m

▪ Static magnetic fields above 0.5 mT (5 G)

To determine the suitability of a workplace, one may be tempted to physically measure the field strengths while equipment is in operation. Indeed, this appears to be the conventional approach used by manufacturers of electrical and electronic equipment to respond to inquiries. For example, manufacturers of cellular phones and magnetic stirrers publish maximum field strengths as a function of distance.

However, that approach is not necessarily appropriate for industrial and commercial facilities employing electrical equipment. The steady-state field strengths in these environments may be greatly exceeded during power system transients, such as faults.

Magnetic Field Calculation

Let us assume a single conductor of infinite length. For discussion purposes, what AC current will produce a magnetic field of 0.1 mT at a distance of 1 m? The magnetic field density, B, in weber/m² or telsa, is given by:

B = (µ₀ I) / 2πr, where

µ₀ = permeability = 4π x 10^-7 h/m

I = current (A)

r = distance from conductor (m)

Setting B = 0.1 mT and solving for current:

I = 500 A

Thus as little as 500 A (60-Hz, rms) will exceed the suggested maximum flux guideline one meter away from the conductor. Geometrically, this arrangement is not at all far fetched. The equivalent of a single conductor, with no cancellation by return current or other phases, can occur during phase-to-ground faults or, during steady state conditions where phases are physically separated—such as at motor terminals, end windings, etc.

In any case, the available fault current on many low- and medium-voltage 60-Hz circuits will greatly exceed 500 A. And, of course, it would be totally impractical to limit access to such areas.

In conclusion, it appears that little or no consideration is being given in the electrical power industry and in cardiology to the risks posed to pacemaker wearers by electrical power transients.

© Doug Criner 2002