Electric, Fuel Cell and Hybrid vehicles may contain many types of high voltage systems. Adequate barriers between occupants and the high voltage systems are necessary to provide protection from potentially harmful electric current and materials within the high voltage system that can cause injury to occupants of the vehicle during a crash. This SAE Recommended Practice is applicable to all Electric, Fuel Cell and Hybrid vehicle designs that are comprised of at least one voltage bus with a nominal voltage greater than or equal to 60 Volts DC or 30 VAC. This Recommended Practice addresses electrical isolation integrity, electrolyte spillage, and retention of the battery system.

Purpose

The purpose of this document is to define test methods and performance criteria which evaluate high voltage system spillage, battery retention, and electrical system isolation in Electric, Fuel Cell and Hybrid vehicles during specified crash tests.

Field of Application

The vehicles covered in this document are Electric, Fuel Cell and Hybrid vehicles with a gross vehicle weight of 4536 kg (10 000 lb) or less and whose speed attainable in 1.6 km on a paved level surface is more than 40 km/h.

Product Classification

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Form

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Rationale

Time

Previous revisions of this document did not include a time limit in which the vehicle had to comply to the test procedure. The 5 second time frame was chosen as a reasonable system response time that also protects vehicle occupants and bystanders. UL 2202 Section 6.2, Stored Energy, references this time as the time to measure voltage across capacitors after energy has been removed.

Voltage

The addition of the voltage criteria in this document provides an alternative means of shock protection by discharging high voltage to non-harmful voltage levels.

Isolation

The change to allowable isolation of fuel cells is based on recognition that fuel cell stacks, unlike batteries, operate only as DC systems that are not charged or back-fed.

The allowable impedance for Electric Vehicles (EV) was based on the possibility that EVs may contain or be connected to utility power grids for the purpose of charging batteries. As such, the requirement for EVs was conservatively set to 500 ohms per volt based on safety characteristics of AC systems. Given that fuel cell hybrid vehicles will likely contain battery systems, power converters and motor controllers, and traction motors similar to the original EVs, changes to existing isolation requirements for high voltage AC (or potentially AC charged) systems are not necessary or appropriate; however, requirements for new DC systems in fuel cell vehicles need to be examined with regard to safety.

The selection of 100 ohms/volt as total impedance is consistent with international standards for electric vehicles and electrical equipment.

a. ISO 6469-3 (working group of TC22/SC21) indicates that the total allowable impedance to chassis from isolated loads is 100 ohms per volt for Evs and drafts of fuel cell electrical requirements.

b. IEC curves for allowable AC and DC both allow 100 ohms per volt.

Fuel cell vehicles have aqueous coolant in direct contact with the fuel cell active area. The conductivity of this coolant is a key factor in the isolation characteristics of a fuel cell. Coolant conductance increases with time which decreases isolation. It is expected that fuel cells will maintain an isolation of 125 ohms/volt with aged coolant. With the remaining high voltage system maintaining an isolation of 500 ohms/volt, the total high voltage system isolation shall not be less than 100 ohms/volt (1/125 ohms/volt + 1/500 ohms/volt = 1/100 ohms/volt).

It is assumed that the vehicle complies with these isolation criteria in normal operation.

Energy

Energy is based on voltage, current and the duration of the voltage-current flow. If energy, time and current are sufficiently low then voltage can be quite high and not result in a hazard. This is what occurs in a typical shock generated from static build up.

Using IEC 479-1 as a guide, a body current of 200 mA for 10 msec yields the lowest energy level that may produce a shock hazard. (Here a shock hazard is defined to be a body current / time duration that is in the DC-3 zone defined in IEC 479-1.) Assuming this minimum as an upper safety bound. Figure 1 below shows Current Duration versus Voltage at 200 mA for various energy levels. Using the target of 200 mA for less then 0.01 seconds as an upper safety bound, an energy of 0.4 J would satisfy the target values. A factor of two is included yielding the final energy limit of 0.2 J.

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