Contact Current
How Conductive Objects Concentrate RF Current Into the Body
When a large metallic object sits in the near field of a powerful HF or VHF transmitter, the time-varying electric field drives displacement currents that charge the object and turn it into an unintentional re-radiator. If the object is poorly grounded, it floats at an RF potential relative to true earth. A person who then touches that object becomes the low-impedance path to ground, and the stored RF energy discharges through the hand, arm, torso, and feet. The current that flows in this scenario is the contact current, and it is regulated separately from whole-body induced current because the contact geometry concentrates the current into a very small skin area.
The severity of contact current depends on body impedance, which is frequency dependent. The IEC and IEEE body-equivalent network approximates a grasping human as roughly 1.5 kΩ of resistance in series with a small capacitance, so the magnitude of the body impedance falls toward the resistive floor as frequency rises. Above about 100 kHz the hazard transitions from nerve and muscle electrostimulation (which dominates at power-line frequencies) to localized tissue heating, which is why the regulatory limits become flat with frequency in the 100 kHz to 110 MHz band rather than continuing to scale. At a fingertip contact patch of only a few square millimeters, even 67 mA produces a current density high enough to cause a painful thermal burn.
Grasping contact, where the whole hand wraps a conductor, presents lower body impedance and a larger contact area than touch contact, where only a fingertip bridges the gap. Counterintuitively the regulatory grasping limit (100 mA) is higher than the touch limit (67 mA) because the perception and pain thresholds differ with contact configuration, not because grasping is inherently safer. Field technicians measure both cases. Mitigation is almost always bonding the offending object to the station ground system so that no RF potential can build up.
Governing Relationships
Icontact = Vobj / |Zbody|, |Zbody| = √(R2 + (1 / (2πfC))2)
Localized tissue heating (SAR-like):
Slocal ≈ J2 / (σ × ρ) W/kg, J = Icontact / Acontact
Power dissipated at the contact:
P = Icontact2 × Rcontact
Where Vobj = RF open-circuit voltage on the object, R ≈ 1.5 kΩ = body-model series resistance, C = the series capacitance, f = frequency, J = current density, Acontact = contact area, σ = tissue conductivity, ρ = tissue density. Example: 67 mA spread over a 3 mm2 fingertip patch gives J = 0.067 A / (3×10-6 m2) ≈ 2.2×104 A/m2 (0.022 A/mm2), a density high enough to drive a localized thermal burn.
Contact Current Limits by Standard and Tier
| Standard / Tier | Frequency | Grasping Contact | Touch Contact | Basis |
|---|---|---|---|---|
| IEEE C95.1 Upper Tier (occupational) | 100 kHz to 110 MHz | 100 mA | 67 mA | Thermal / RF burn |
| IEEE C95.1 Lower Tier (public) | 100 kHz to 110 MHz | 45 mA | 33.5 mA | Thermal / RF burn |
| FCC OET 65 (controlled) | 100 kHz to 100 MHz | 100 mA | 67 mA | Adopts IEEE upper tier |
| FCC OET 65 (uncontrolled) | 100 kHz to 100 MHz | 45 mA | 33.5 mA | Adopts IEEE lower tier |
| Below 100 kHz | 3 kHz to 100 kHz | frequency-scaled | frequency-scaled | Electrostimulation |
Frequently Asked Questions
What is the difference between contact current and induced body current?
Induced body current flows through a person standing in an RF field with no physical contact and is dominated by the ankle, limited to 100 mA per foot for occupational exposure. Contact current flows when that person touches a separate conductive object charged by the field, completing a circuit to ground through the body. Contact limits are lower because the small fingertip or hand contact area concentrates current density into a burn or perception hazard. From 100 kHz to 110 MHz the occupational grasping limit is 100 mA and the touch limit is 67 mA.
What are the IEEE C95.1 contact current limits between 100 kHz and 110 MHz?
Above 100 kHz the limits are flat because the hazard shifts from electrostimulation to thermal heating. The upper-tier (occupational) maximum is 100 mA grasping and 67 mA touch; the lower-tier (general public) maximum is 45 mA grasping and 33.5 mA touch. Below 100 kHz the limits scale with frequency because nerve and muscle stimulation thresholds rise as frequency drops. The FCC adopts the same values in OET Bulletin 65 for evaluating compliance near broadcast and high-power HF or VHF transmitters.
How is RF contact current measured in the field?
Technicians use a clamp-on RF current probe or an in-line contact-current meter that mimics the impedance of the human body, typically the 1.5 kΩ series resistance and capacitance of the IEC equivalent circuit, sometimes called a phantom hand. A metallic test electrode is brought into contact with the suspect object while the meter reads the RF current driven through the body-equivalent network to ground. Both touch and grasping cases are checked, and any object exceeding the limit is bonded to ground or fitted with insulating barriers.