How Electrical Bonding Can Save Your Life: EV Safety Explained

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.

FREQUENTLY ASKED QUESTIONS

What is the purpose of the NASA standard for electrical bonding in launch vehicles, spacecraft, payloads, and equipment?

The NASA standard for electrical bonding aims to establish essential guidelines for electrical connections in launch vehicles, spacecraft, payloads, and related equipment. This standard’s main objective is to ensure that these aerospace components have reliable and effective electrical bonds, which are categorized based on their specific functions. By defining these requirements, the standard helps prevent electrical failures and enhances overall safety and performance.

Moreover, the document outlines different classes of electrical bonds, detailing distinct requirements for each class. To aid understanding, it provides explanations for the rationale behind particular specifications. Additionally, the standard offers supplementary information that can assist in customizing electrical bonding solutions for new or unique applications.

How can lessons learned be associated with the NASA electrical bonding standard?

How to Link Lessons Learned to the NASA Electrical Bonding Standard

Integrating lessons learned with the NASA Electrical Bonding Standard is a straightforward process that ensures all insights are effectively utilized. Here’s how to do it:

1. Identify Relevant Lessons:
   • Start by searching for lessons related to the electrical bonding standard in the designated lesson repository. These lessons are typically pre-evaluated to ensure they meet quality standards.
2. Creating a New Record:
   • Once you’ve found a relevant lesson, initiate the process by selecting the option to “Create a new Lessons Learned record for this Standard.” Follow the prompts to enter all necessary details, ensuring nothing is overlooked.
3. Adding New Insights:
   • If you identify a lesson learned that hasn’t been documented, you can contribute new insights. Begin by accessing the preferred network or platform for creating lessons related to this standard.
4. Approval and Vetting:
   • Any new lesson will need to go through a vetting process. This ensures that all documented lessons maintain a high quality and relevance standard. Once approved, they can be associated with the bonding standard.
5. Finalizing the Association:
   • Once your lesson is vetted and approved, finalize the process by selecting the option to “Create a new Lessons Learned record for this Standard” again and linking the new lesson.

By following these steps, you’ll ensure that valuable knowledge is effectively captured and aligned with the electrical bonding standards, leveraging collective insights to enhance future work.

Are there any application notes associated with the NASA electrical bonding standard?

Are There Any Application Notes for the NASA Electrical Bonding Standard?

In reviewing the NASA electrical bonding standard, you’ll find no specific application notes linked directly to it. While the standard itself provides comprehensive guidelines and criteria, there aren’t any supplementary documents or application notes available.

For a deeper understanding, it may be helpful to explore related resources or third-party publications, such as those from industry leaders like IEEE or SAE International, which offer additional context or case studies that complement the guidelines of this standard.

What is the document scope of the NASA electrical bonding standard?

Document Scope of the NASA Electrical Bonding Standard

The NASA electrical bonding standard outlines foundational criteria for the electrical bonding of launch vehicles, spacecraft, payloads, and other equipment. This standard serves several purposes:

• Purpose Definition:It establishes fundamental requirements for aerospace electrical bonding, ensuring a unified approach across all associated systems and equipment.
• Classification of Bonds:Electrical bonds are categorized based on their specific functions or roles within the system, enabling clear guidelines for each category.
• Requirement Specification:Detailed requirements are provided for various bond classes, ensuring clarity and consistency in application.
• Rationale and Tailoring:Whenever possible, the reasoning behind each requirement is explained. The document also includes additional information to assist in adapting these requirements for new applications, as necessary.

Overall, this standard serves as a comprehensive guide, supporting consistent and effective electrical bonding practices in aerospace contexts.

How is the NASA electrical bonding standard classified in terms of technical discipline?

The NASA electrical bonding standard falls under the comprehensive umbrella of technical disciplines. Specifically, it is classified within the following categories:

• Electrical and Electronics Systems: This includes all aspects related to electrical circuitry, connections, and bonding to ensure safety and functionality.
• Avionics/Control Systems: Covering the electronic systems used for aircraft and spacecraft, this discipline ensures precise control and operation.
• Optics: This area involves the study and use of light, crucial in systems where visual and sensor-based guidance is necessary.

These categories highlight the standard’s scope, covering essential systems that ensure efficient and reliable performance in aerospace applications.

Is the NASA electrical bonding standard publicly accessible?

Yes, the NASA electrical bonding standard is available for public access on the internet. Here’s a breakdown of the key points:

• Accessibility: This document is cleared for online distribution, meaning anyone with internet access can view and utilize it.
• Distribution Authorization: It is categorized for unrestricted public release, allowing both individuals and organizations to access it without special permissions.

In summary, the document is published with the intent to be freely accessible, making it easy for researchers, engineers, and interested parties to find and reference.

Is the NASA electrical bonding standard a mandatory standard?

When delving into the specifics of electrical bonding standards in aerospace, particularly those linked to NASA, it’s essential to clarify whether they are mandatory. A “mandatory standard” typically refers to regulations or guidelines that must be adhered to without exception, ensuring safety, reliability, and consistency across projects and systems.

Understanding the Scope

1. Regulatory Context: Standards developed by large aerospace organizations often serve as benchmarks for safety and performance. However, whether these standards are obligatory can depend on the context of their application.
2. Project Requirements: For government contracts or projects directly under NASA’s purview, adherence to their specified standards could be seen as required. This ensures compatibility and safety across collaborative missions and technologies.
3. Third-Party Applications: Companies or contractors working independently or with other organizations might choose to follow these standards as best practices. Still, these may not be legally binding unless specified by a particular contract.

Industry Best Practices

• Safety and Quality Assurance: Many companies adopt NASA’s standards voluntarily due to their rigorous approach to safeguarding operations.
• Competitive Edge: Aligning with recognized standards can enhance a company’s reputation, making them more attractive to potential partners and customers.

In summary, while NASA’s electrical bonding standards significantly influence aerospace practices, their mandatory status can vary based on contractual obligations and project specifics. Always review contractual terms and project documentation to determine the necessity for compliance.