EV.5.4 Accumulator Isolation Relays - AIR

EV.5.4.1 Every Accumulator Container must contain minimum one fuse (EV.6.6) and two or more Accumulator Isolation Relays (AIR)

Photo - AIR(+)

Each accumulator container must be equipped with at least two Accumulator Isolation Relays (AIR's), one for each pole of the Accumulator. These contactors must be relays which are powered by the shutdown circuit. This is the case because any fault in the shutdown circuit (or case in which the vehicle is powered off, of course) should lead to an interrupt in the power supplied to the AIR's which opens and disconnects their battery poles. Shown to the left is our ALTRAN Magnetics Relay, used to isolate the positive terminal of our Accumulator.

In addition, the Accumulator must be fused in at least once location. Rules for this are supplied in EV.6.6, and summarized below.

Reference Rules - EV.6.6

EV.6.6 Overcurrent Protection

EV.6.6.1 All electrical systems (both Low Voltage and High Voltage) must have appropriate Overcurrent Protection/Fusing.

EV.6.6.2 Unless otherwise allowed in the Rules, all Overcurrent Protection devices must:

a. Be rated for the highest voltage in the systems they protect.

Overcurrent Protection devices used for DC must be rated for DC and must carry a DC rating equal to or more than the system voltage

b. Have a continuous current rating less than or equal to the continuous current rating of any electrical component that it protects

c. Have an interrupt current rating higher than the theoretical short circuit current of the system that it protects

EV.6.6.3 Each parallel element of multiple parallel battery cells, capacitors, strings of battery cells, strings of capacitors, or conductors must have individual Overcurrent Protection.

EV.6.6.4 Any conductors (wires, busbars, etc) conducting the entire pack current must meet one of:

a. Be appropriately sized for the total current that the individual Overcurrent Protection devices could transmit

b. Contain additional Overcurrent Protection to protect the conductors

EV.6.6.5 Battery packs with Low Voltage or non voltage rated fusible links for cell connections may be used when all three conditions are met:

An Overcurrent Protection device rated at less than or equal to one third the sum of the parallel fusible links and complying with EV.6.6.2.b above is connected in series.
The AMS can detect an open fusible link and will Open the Shutdown Circuit EV.7.2.2 if a fault is detected.
Fusible link current rating is specified in manufacturer’s data or suitable test data is provided.

EV.6.6.6 If conductor ampacity is reduced below the ampacity of the upstream Overcurrent Protection, the reduced conductor longer than 150 mm must have additional Overcurrent Protection.

This additional Overcurrent Protection must be:

a. 150 mm or less from the source end of the reduced conductor

b. On both positive and negative conductors in the Tractive System

c. On the positive conductor in the Grounded Low Voltage System

EV.6.6.7 Cells with internal Overcurrent Protection may be used without external Overcurrent Protection if suitably rated.

Most cell internal Overcurrent Protection devices are Low Voltage or non voltage rated and conditions of EV.6.6.5 above will apply.

As stated by EV.6.6.1, there must be Overcurrent Protection (inclusive of Fusing) for both Low Voltage and High Voltage components. Furthermore, EV.6.6.2 stipulates that Overcurrent Protection devices must match or exceed the system voltage and have a current rating equal to or greater than the components they protect. They must also have an interrupt current rating higher than the system's short circuit current.

According to EV.6.6.3, each parallel element (cells, capacitors, conductors) must have individual Overcurrent Protection. EV.6.6.4 states that conductors carrying pack current must be appropriately sized or have additional protection. Battery packs with specific fusible link configurations are allowed under certain conditions, including the use of series-connected Overcurrent Protection devices as stated by EV.6.6.5.

As described by EV.6.6.6, reduced conductor ampacity requires additional Overcurrent Protection within 150 mm from the source end. Finally, EV.6.6.7 allows for cells with internal Overcurrent Protection to be used without external protection if they meet specified criteria.

EV.5.4.2 The Accumulator Isolation Relays must:

a. Be a Normally Open type

b. Open both poles of the Accumulator

Rules Reference - T.9.1.3

T.9.1.3 Normally Open

A type of electrical relay or contactor that allows current flow only in the energized state

A Normally Open Type, as referenced in EV.5.4.2.a (and defined in T.9.1.3) also known as a normally open switch, is a switch or contact that prevents electricity from flowing when it's not activated or compressed. When the switch is activated, it closes the circuit and allows electricity to flow. Once the input energy is removed, the circuit returns to the open position and electricity stops flowing.

And as for why both poles of the Accumulator must be opened by the Accumulator Isolation Relays (EV.5.4.2.b), the goal is to prevent backflow. When the relay is open, no current can flow in or out of the system and prevents uncontrolled charge or discharge. Ultimately, the goal is to physically disconnect the accumulator to mitigate the issue of accidentally activating components or letting current flow freely.

EV.5.4.3 When the AIRs are open, High Voltage T.9.1.1 must not be external of the Accumulator Container

Rules Reference - T.9.1.1

T.9.1.1 High Voltage – HV

Any voltage more than 60 V DC or 25 V AC RMS

The Accumulator Isolation Relays are of the Normally Open Type, so in the non-energized state, there should be no High Voltage outside of the Accumulator Container. High Voltage, as stated in T.9.1.1 is defined as DC voltage greater than 60V, or AC voltage greater than 25VRMS.

EV.5.4.4 The Accumulator Isolation Relays and any fuses must be separated from the rest of the Accumulator with an electrically insulated and Nonflammable Material (F.1.18).

The Accumulator operates within a defined high voltage whose potential can cause electrical arcing or short circuits if not properly insulated. Fuses and isolation relays are designed to interrupt the flow of current under fault conditions (such as overcurrent or short circuits). If these components are not insulated properly, they could unintentionally conduct electricity between the Accumulator poles. The Nonflammable Material is defined in F.1.18, and explained below.

Rules Reference - F.1.18

F.1.18 Nonflammable Material

Metal or a Non Metallic material which meets UL94-V0, FAR25 or approved equivalent

This rule specifies the flammability characteristics of metallic and non-metallic (often thermoplastic) materials to ensure that materials used extinguish potential fires in a timely and safe manner.

Table - UL94 Ratings

UL94, the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing, is a plastics flammability standard which determines the material's tendency to either extinguish or spread the flame once the specimen has been ignited.

Table - FAR25 Testing

FAR 25.253 is a standard of the Federal Aviation Administration (FAA) for determining the flammability characteristics of materials & components used in the aircraft. The purpose is to establish repeatable, reproducible, easy test methods to assess potential fire risks that may be happened in the aircraft.

EV.5.4.5 A capacitor may be used to hold the AIRs closed for up to 250 ms after the Shutdown Circuit Opens EV.7.2.2

In some cases, teams may have other safety features working synchronously with the AIR's to preserve their hardware. It could potentially be taxing on equipment such as inverters to be subject to a driver pressing E-Stops continuously for testing. For specific inverters such as the BAMOCAR D3, leaving the AIR's closed for an additional time under 250 milliseconds would allow for the Rotary Field Enable (RFE) to disable, and provide a window to disengage the pulse width modulation going through the motor and tractive system before engaging the shutdown circuit.

Rules Reference - EV.7.2.2

EV.7.2.2 When the Shutdown Circuit Opens:

a. The Tractive System must Shutdown

b. All Accumulator current flow must stop immediately EV.5.4.3

c. The voltage in the Tractive System must be Low Voltage T.9.1.2 in five seconds or less

d. The Motor(s) must spin free. Torque must not be applied to the Motor(s)

According to EV.7.2.2, opening the Shutdown Circuit requires that components within the Tractive Path must be shut down. Additionally, current flow must stop within the Accumulator and the Accumulator Isolation Relay (AIR) Coils must be engaged. As stated in EV.5.4.3, the Accumulator Isolation Relays are of the Normally Open Type, so in the non-energized state, there should be no High Voltage outside of the Accumulator Container.

Additionally, the Tractive system, within five seconds of the Shutdown Circuit engaging, must be read on a multimeter as Low Voltage, which, stated in T.9.1.2 is defined as DC voltage less than 60V, or AC voltage less than 25VRMS. Additionally, torque cannot be applied to slow down or speed up the motor(s).

Reference Rule - T.9.1.2 (Referenced in EV.7.2.2)

T.9.1.2 Low Voltage - LV

Any voltage less than and including 60 V DC or 25 V AC RMS

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