Download EVS-03-04 Comments on Protective Barrier Option for Post

EVS-03-04
Comments on Protective Barrier Option for Post-Crash Electrical Safety
In the final rule (76 FR 45436 published on July 29, 2011) responding to petitions for
reconsideration of the June 14, 2010 final rule, NHTSA stated that it was uncertain whether indirect
contact failure modes would be sufficiently accounted for by the physical barrier compliance option and
that the research it initiated to evaluate this issue has not been completed. The research has now been
completed and the report is being finalized.
The Hydrogen vehicle GTR offers physical protection from direct contact and indirect contact as
a standalone option for electrical safety post-crash without the need for either the absence of high voltage
or electrical isolation. The analysis conducted by the researcher indicates that protective barriers alone
may not limit currents through a contacting body to acceptable levels and that complimentary electrical
isolation is needed for electrical safety. The analysis conducted showed various scenarios where a
protective barrier was insufficient for ensuring shock protection when there was loss in electrical isolation
in a crash. Some of these scenarios are shown in Figures 1, 2, and 3. In these figures, RChL, RChH, RCh are
chassis resistance, Rb is body resistance, and Vs and Rs are the the high voltage source voltage and
resistance, respectively. RiH and RiL are the electrical isolation resistances between the positive and
negative side of the high voltage source and the chassis, respectively.
Figure 1. Electrical isolation is lost at both the positive and negative side of the high voltage source and a
person contacts the protective barriers.
Figure 2. Loss in electrical isolation in positive and negative side of two protective barriers and contact
with chassis or the protective barriers.
Figure 3: Loss of electrical isolation in protective barriers and loss of electrical chassis bonds and contact
with both protective barriers.
The researchers also developed a simple model to illustrate electrical safety under loss of electrical
isolation, such as the condition shown in Figure 4. In the body current versus short circuit resistance
graph in Figure 4, the body current lines will move significantly depending upon voltage, source
resistance (R43), chassis resistance (R44), and body resistance (R45).
Figure 4. Body current for various short circuit and body resistance values for a high voltage source of
300 V.
This simple circuit analysis illustrates that safety is based upon electrical resistance of chassis connections
being lower than body resistance. The HV GTR requires the chassis resistance to be less than 0.1 Ohm.
The figure illustrates that body current is dependent upon parameters other than chassis resistance alone.
For this model and the parameters selected, low body resistances (500 ohms to 50,000 ohms) may result
in body currents above the IEC TS 60479 recommended 2mA.
In summary, the research evaluating protective barrier as a standalone option for post-crash electrical
safety showed that:

Protective barriers may not limit currents through a contacting body to acceptable levels without
complimentary electrical isolation.
- Electrical isolation may be necessary in conjunction with conductive barriers to limit
currents through the contacting body.

For indirect contact the body is electrically in parallel with the isolation resistance
- Current through the body is not limited and does not provide the same level of protection as
a series circuit
- May activate electrical fuses or current limiting devices when barrier isolation is lost

Currently analysis could not confirm the requirement that a chassis resistance less than or equal
to 0.1 Ω is sufficient to ensure that body currents are within acceptable levels in the event of an
indirect contact exposure.
- More information is needed concerning the engineering rationale and basis for this
requirement in the standards.
In the failure scenarios illustrated in Figures 1-3 where the protective barrier option alone would not be
effective in preventing electric shock, at least three failure events are needed to occur – loss in electrical
isolation at two locations in the powertrain, and the event of someone contacting the energized chassis at
two locations to complete the circuit. The agency seeks information on the likelihood of the occurrence
of such multiple failure events in crashes involving electric powered vehicles.
Comments on Energy Option for Post-Crash Electrical Safety
In the final rule published on June 14, 2012 (75FR33515), NHTSA responded to the Alliance/Global
Automakers comments on including an energy option in FMVSS No. 305. The agency stated that it was
not pursuaded that a low-energy option of 0.2 Joules is adequate for ensuring electrical safety. We are
reiterating our position of the energy option for electrical safety as outlined in the June 2012 final rule.
The agency conducted its own analysis using Figures 20 and 22 of IEC TS 60479–1(Figures 5 and 6) and
several permutations of body current, body resistance, current duration, and safety factors to try to
determine safe energy limits. Based on that analysis, the agency determined that applying different
permutations of voltage levels, body resistance, shock duration, and factor of safety can result in different
safe energy levels, some of which are less than the 0.2 J energy level specified in SAE J1766. Given that
the IEC report indicates that the lowest human body impedance is a resistance of 500 ohms, and that the
boundary between zones DC–2 and DC–3 is 200 mA of body current for 10 msec shock duration, we
determined the corresponding amount of voltage through the Ohm’s Law equation Current (A) *
Resistance (ohms) = Voltage (V), in this case, 0.2 A * 500 ohms = 100 V. We then applied the same
method used in SAE J1766 to calculate energy for a 10 msec shock duration with the equation Voltage
(V) * Current (A) * Time (s) = Energy (J), in this case, 100 V * 0.2 A * 0.01 s = 0.2 J (or, equivalently,
200 mJ). The SAE applied a safety factor of 2. Doing the same for 200 mJ, the agency concluded that a
safe energy level would correspond to 100 mJ—half the energy level specified in J1766 and
recommended by Alliance/AIAM. Following the same procedure, for a voltage source of 65V and body
resistance of 500 ohms, the body current was 130 mA, and the corresponding energy was 84.5 mJ.
Applying a factor of safety of 2, the safe energy level was 42.2 mJ. Even without applying a factor of
safety, the energy level is less than half of that recommended by the Alliance/AIAM. Based on this
analysis, the agency concluded that the assumptions associated with voltage, current, and shock duration
used to derive the proposed limit of 0.2 J for the energy option are not well supported.
Figure 5. Conventional time/current zones of effects of AC. currents (15 Hz to 100 Hz)
(from IEC TS 60479)
Figure 6. Conventional time/current zones of effects of DC currents (from IEC TS 60479).
Based on their assumptions, the Alliance’s approach to determine minimum allowable energy levels
presented in the June 2009 supplemental comments would allow body currents of 20 mA for shock
duration of 1 second, 28 mA for shock duration of 0.5 seconds, and 200 mA for shock duration of 10
msec. In contrast, the electrical isolation option of FMVSS No. 305 does not allow more than 10 mA of
DC body current and 2 mA AC body current at either 1 second, 0.5 seconds, or 10 msec shock duration.
The agency thus believes that the low-energy option would result in a higher risk level than that provided
by the electrical isolation option of FMVSS No. 305, which protects against involuntary muscular
contractions without any harmful physiological effects. Moreover, no technical basis has been provided
for the assumption that for current durations less than 2 seconds, no serious damage is observed with
sufficiently low energy even if the current passing through the body is relatively high. The IEC charts
clearly indicate that shock duration for one second is sufficiently long to cause involuntary muscular
contractions, which are currently mitigated through the electrical isolation requirement of FMVSS No.
305.
Given the results of our analysis which shows that the energy option requested by commenters would be
less stringent and pose a greater risk of electric shock hazard than the electrical isolation option, the
agency is not including the low-energy option as a method for electrical safety.