What is the NESC "5-mA Rule"?

The "5mA-Rule" is a current limit listed in the National Electrical Safety Code® [1]. The rule states:

For voltages exceeding 98 kV ac to ground, either the clearances shall be increased or the electric field, or the effects thereof, shall be reduced by other means as required to limit the steady-state current due to electrostatic effects to 5 mA if the largest anticipated truck, vehicle, or equipment under the line were short-circuited to ground. The size of the anticipated truck, vehicle, or equipment used to determine these clearances may be less than but need not be greater than that limited by federal, state, or local regulations governing the area under the line. For this determination, the conductors shall be at a final sag at 50 °C (120 °F).”

See rules 232.C.1.c, 232.D.3.c, 234.G.3.

What are electrostatic effects?

Electrostatic effects refer to electromagnetic conditions in which the frequency and current are low enough that magnetic field effects can be neglected. Therefore, energy exchange is due to electric field effects. In the context of the 5-mA Rule, energy is exchanged in the electric field between the high voltage conductors and the surface of the metallic vehicle or object. A person contacting the vehicle becomes a path to ground for this current. The situation can be represented by an equivalent circuit in which current flows through the capacitance between the transmission line conductors and the vehicle and from the vehicle to ground through ther person. Since the rule calls for a short circuit path to ground, human impedance and object capacitance to ground are neglected.

Where did the 5 mA limit come from?

A detailed history of research leading to the 5mA threshold is available in [2]. In short, various research efforts dating back to the 1930's were performed to develop understanding of the physiological impacts of electricity. Some studies involved carefully controlled experiments in which volunteers were exposed to increasing levels of 60 Hz current to establish a "let-go" threshold. This is the maximum current at which a person is still capable of releasing an electrode they have grasped. Voluntarily releasing the electrode may not be possible above the let-go threshold due to muscle contractions. The figure below summarizes the data from [3].

Is electrostatic contact current the same as static discharge?

No. The 5-mA Rule refers to a continuous AC current. A static discharge is a very short duration current that equalizes a charge difference between two electrodes. It is important to understand that the electrosatic contact currents referred to in the NESC 5-mA rule are continuous until the person breaks contact with the vehicle or object.

Is 5 mA of current perceptible?

Yes, in fact 5 mA may be painful. The table below from [4] lists typical biological effects of varying 60 Hz current levels in an average size man. As with most electrical risks, statistical anomalies could result in a more serious medical response than those listed below.

Do other industry sources recommend limits other than 5 mA?

Yes. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) recommends the following guidelines based on perception in adults and children [5]:

Can conditions exceed 5 mA for a transmission line that complies with the 5-mA Rule?

Yes. Note that the NESC statement under the first question above refers to specific conductor sag conditions associated with the limit. If the line is electrically loaded such that conductor temperature results in sag that exceeds these conditions, the resulting contact current might be greater than 5 mA.

How can electrostatic contact currents be reduced?

Electrostatic contact currents can be reduced by the following means:

• Increase the height of the conductors.
• Use bundle/phase configurations that have better field cancellation and reduce electric fields at ground level.
• Install shield wires below the phase conductors.
• Ground the object (immobile objects). This shunts current away from the person touching the object.
• Limit object or vehicle size or orientation with respect to the overhead line.

What are some important considerations for 5-mA Rule analysis?

• Consider object position under the line. The electric field varies as a function of position within the right-of-way. Vehicles or objects position under the peak of the field will have a higher contact current that at other locations. Physical constraints may prevent positioning under the region of maximum electric field.
• Consider object orientation (parallel or perpendicular) with respect to the line. An object with length greater than width will have a higher contact current when oriented parallel to the line since more of it will be within the influence of higher electric field values.
• Consider using the maximum operating voltage of the line rather than the nominal voltage.
• If multiple overhead line circuits are involved, you can take advantage of certain phasing configurations that will improve field cancellation at ground level. However, if the phase angle relationships between the circuits vary or are unknown, then worst case phase configurations may need to be assumed.
• Body resistance and shoe resistance are typically neglected since the NESC Rule refers to "short circuit" currents to ground.

Where can I find information about state vehicle size limits?

State transporation departments publish vehicle size limits. A convenient federal site summarizing state publications is at the link below. After clicking the link, scroll down past the initial transmittal letters to see links to the state limits. Link: Compilation of Existing State Truck Size and Weight Limit Laws