Wired for Safety: The Science Behind Electrical Bonding in Electric Vehicles

How Electrical Bonding Can Save Your Life

As cooler months descend upon us, so do tacky wool sweaters. If you’ve had the opportunity to wear one for an ugly sweater party, you may have also experienced becoming a conduit for unexpected static shock as you grabbed the serving spoon in the punch bowl. While unpleasant, this phenomenon can teach us a great deal about electric vehicle safety systems that can keep builders or repairers working on them safer from electric shock.

What is Potential Equalization?

When you felt that jolt of electricity passing through your hand and onto the metal serving spoon, you were experiencing a “potential equalization” across two different materials. You—and the thing you touched—were at two different electric potentials, or “voltages.” When you and that spoon got close enough together, the shock you felt was the differing potentials becoming equalized. Had you and the spoon been at the same electric potential, the shock would have never occurred.

To protect EV components and the people working on them, potential equalization is essential.  Bonding is a type of potential equalization where one intentionally electrically connects all exposed metal items not designed to carry electricity as protection from electric shock by keeping these metals all at the same electric potential. In the context of EVs, this includes conductive components like the chassis, frame, body, and external housings. This prevents potential differences that could occur due to different components being connected to different electrical sources or having different resistances. Through potential equalization, we can minimize the risk of electric shock, arc formation, and electrical damage within the vehicle.

In many cases, bonding is achieved by simply mounting the conductive housing of a high-voltage component directly onto the metallic frame, chassis, or firewall of the vehicle.

What Does Electrical Bonding Look Like in an EV?

Electrical bonding can come in different forms. In many cases, bonding is achieved by simply mounting the conductive housing of a high-voltage component directly onto the metallic frame, chassis, or firewall of the vehicle.

In other cases, a bonding cable may instead be run from the high-voltage component housing to the chassis. The ReVolt crate motor has a bonding strap that connects the main body of the motor to the chassis of the car.

The Cascadia Motion inverter has a marked hole with a chassis ground symbol in the top right hand corner to indicate the placement of a bonding cable.

Often, you will find both bonding approaches in a vehicle. In the EV training bench, all component housings are bolted to the metal tabletops. Bonding straps are then used to bond the different parts of each bench together.

How Does Bonding Keep Us Safer from Loss of Isolation Faults?

EVs, just like gas cars, have low-voltage circuitry that grounds to the chassis. The high-voltage circuitry, however, is a “floating system,” in that its own ground is completely isolated from the chassis. Any high-voltage circuitry will be covered in electrical insulation that creates a substantial degree of resistance, often on the order of megohms (millions of ohms). In the case of an accident where the housing or insulation from the high-voltage system became damaged, the HV leakage would energize whatever it touched to the same potential. So even if a technician isolated the HV system, they could still be at risk of making contact with two deadly differences in potential. With proper bonding, however, that damaged housing would bring anything else it is bonded to at the same potential, limiting the chances of such an encounter.

In addition to limiting the amount of materials with different electric potentials, good bonding allows devices, like ground fault monitors, to check component housings and the chassis for high-voltage leaks. If a high-voltage component is not properly bonded, or the cable it is bonded to has oxidized over time, the ground fault monitor would not be able to notice that a dangerous high-voltage leak was occurring.

How Can You Check Bonding?

In addition to visually inspecting bonding cables and mounting points for corrosion, dirt, and poor contact, OEMs may provide instructions to check bonding using a special tool called a milliohm meter. The Hioki REM3548 portable DC resistance meter is a common tool used in shops involved in EV collision and repair.

Milliohm meter probes have four tips rather than the typical two seen in a digital multimeter. It sends a one-amp current through one probe, while the other measures voltage. This allows it to detect tiny resistance variations on the order of micro-ohms, or millionths of an ohm, resistances that are far too small for a digital multimeter to notice. For example, placing the milliohm meter terminals as close together as possible on a piece of aluminum yielded a resistance of 0.0066 milliohms.

But what kind of resistance values should you be looking for to ensure good bonding? As a baseline, a 1-meter (~3 foot) long cable has an internal resistance of about 1 milliohm. The United Nations Economic Commission for Europe, Regulation 100 (UNECE-R100) provides technical guidance safety requirements with regard to fitting parts on a vehicle with an electric powertrain. It ties the 1-milliohm-per-meter standard as a guiding reference for bonding. If the resistance reading were much higher than this standard, a technician has reason to suspect that the component’s bonding needs improvement.

What Can I Do As a Builder or Shop Owner?

Incorporate bonding cables or mounting points into your design: If you are considering placing HV components on top of insulated materials, like certain paints, wood, carpet, or plastic, make sure you are considering how you will either install a bonding cable or create direct metal contact between your component’s housing and the frame of the vehicle.

Visually inspect components you repair or install: Examine the vehicle for any missing bonding cables or corroded connections that can prevent good bonding.

Use a milliohm meter:  If you are repairing or building an EV for a customer, use a milliohm meter while referencing the manufacturer or international safety standards for bonding.

Documentation: If it wasn’t documented, did it really happen? Take before/after photos of HV components you remove/install and note instances of improper bonding. Collect milliohm meter readings into documents that you can refer back to for that customer.