Kenneth N. Weinstein, Esq.
Mayer Brown LLP
1909 K Street, NW
Washington, DC 20006-1101
Dear Mr. Weinstein:
This responds to your letter requesting an interpretation of Federal motor vehicle safety standard (FMVSS) No. 305, Electric-powered vehicles; electrolyte spillage and electrical shock protection. Specifically, you asked that we confirm that a vehicle will be deemed to be in compliance with S5.3 of FMVSS No. 305 if there is no measurable voltage following the crash tests specified in S6 of the standard. Based on the information you have provided and the analysis below, we agree that where there is no measurable voltage following the crash tests, the vehicle will have passed S5.3 of the standard.
Paragraph S5.3 of FMVSS No. 305, Electrical isolation, currently states that Electrical isolation between the battery system and the vehicle electricity-conducting structure after each test must not be less than 500 ohms/volt. The electrical isolation test procedure, specified in S7.6 of FMVSS No. 305, requires that two voltage measurements be taken after the vehicle is crash-tested: the first measurement (V1) must be taken between the negative side of the propulsion battery and the vehicle chassis, and the second measurement (V2) must be taken between the positive side of the propulsion battery and the vehicle chassis. Electrical isolation is then determined by means of an equation: if V1 is greater than or equal to V2, the equation is:
Ri = Ro (1 + V2/V1 ) [(V1-V1)/V1]
where Ri is electrical isolation, Ro is a known resistance, and V1 is the voltage measured between the negative side of the propulsion battery and the vehicle chassis after the known resistance Ro is inserted between the negative side of the propulsion battery and the vehicle chassis.
If V2 is greater than V1, the equation for measuring electrical isolation is:
Ri = Ro (1 + V1/V2) [(V2-V2)/V2]
where Ri and Ro are the same as above, and V2 is the voltage measured between the positive side of the propulsion battery and the vehicle chassis after the known resistance Ro is inserted between the positive side of the propulsion battery and the vehicle chassis.
These equations are relevant to your question because they both require one voltage measurement to be divided by another. As your letter states, many current electric vehicle designs use electrical contactors to disconnect high voltage sources from the vehicles propulsion system in the event of a crash or other loss of isolation. If the high voltage source is immediately disconnected, there would be no voltage to measure. If there is no voltage to measure, a value of zero could end up in the denominator of an equation used to determine electrical isolation. Mathematically, a value of zero in the denominator of a fraction results in an undefined value, which has no meaning and cannot be used, in this case, to actually calculate electrical isolation. Thus, technically speaking, the equations in S7.6 could not be used to certify compliance with FMVSS No. 305s electrical isolation requirement.
We have decided that a sensible and simple approach is warranted in response to the issue you raise. We do not believe that the mathematically impossible must be a bar against a certification of compliance in this situation. Accordingly, we will interpret FMVSS No. 305 such that a voltage reading of zero will constitute compliance with the electrical isolation requirement.
I hope this information is helpful. If you have any further questions, please do not hesitate to contact Rebecca Yoon of my staff at (202) 366-2992.
Anthony M. Cooke
 NHTSA is currently involved in rulemaking to revise FMVSS No. 305 to allow other ways of determining electrical safety besides electrical isolation, including a requirement that the voltage between the vehicle chassis and the high voltage source be less than 60 VDC or 30 VAC. See notice of proposed rulemaking, 72 FR 57260 (Oct. 9, 2007). As your letter states, this would allow a manufacturer to certify compliance with FMVSS No. 305s requirements if the vehicle achieved a zero post-crash voltage. However, that new requirement has not yet been finalized.
 One could also point out that, mathematically, as your voltage measurement gets smaller and smaller (in other words, as your denominator approaches zero), your isolation becomes infinite. An electrical isolation approaching infinity is clearly greater than or equal to 500 ohms/volt, even if it cannot be defined mathematically.