04.9 EV HV isolation

How do you isolate high voltage systems on an EV?

There are five ways in which high voltage systems on an EV can be shut down:

When airbags deploy* - automatic
Cut loop
Pull plug
Pull fuse
Manual Service Disconnect

*As discussed on the Electrocution Risks page, when airbags deploy in an electric vehicle following a collision, the high voltage systems should automatically isolate.

It's important to note that some EVs don't have a low voltage method to isolate high voltage systems, instead relying on airbag deployment alone.

While automatic HV isolation following airbag deployment should occur, there is no way for emergency responders to test or measure this; ie. we cannot test for current in DC power circuits.

What does HV isolation do?

The high voltage battery pack, components & cables are all contained within an EV, & can pose a risk of electrocution in some circumstances.

When we isolate high voltage systems, all the electricity is drained to the high voltage battery pack underneath the vehicle; this is not an area responders would usually be touching or cutting into for passenger extrication, so isolating HV makes responders safer to work around the EV.

How do EV HV isolation methods work?

To compare each side by side, we've created the following table; click on each heading to read more, see images & a video.

HV isolation following airbags deployment

Isolation should occur automatically, with no action required by emergency responders
Responders cannot detect or measure whether isolation has occurred in an electric vehicle's DC circuits
Follow the manufacturer emergency response guide (ERG) & your agency SOPs on appropriate PPC / PPE
- PPC would include full structure gear & self-contained breathing apparatus at a minimum
- PPE would include (at a minimum):
  - Insulated cutting tools

HV will isolate after use of CUT LOOP

A responder cut loop is a LOW VOLTAGE wiring harness that will isolate high voltage systems following a collision or incident involving an EV:

Use the ERG to find cut loop location
- Cut loops are used in all Tesla models & is located under the frunk, just beneath the windscreen to the left or right; responders will need to remove plastic cowling/s to access the cut loop
Cut the loop in two places, completely removing the label
- This ensures the loop cannot accidently reconnect

HV will isolate following use of PULL PLUG

A GREEN pull plug is a LOW VOLTAGE wiring harness & plug that will isolate high voltage systems following a collision or incident involving an EV:

Use the ERG to find pull plug location
- Pull plugs are usually located under the bonnet of the vehicle
Once the pull plug has been it will lock into place & can only be re-engaged with a special manufacturer tool

Note: Green pull plugs are becoming common across most EV brands, excluding Tesla.

HV will isolate after use of PULL FUSE

A pull fuse is a LOW VOLTAGE component that will isolate high voltage systems following a collision or incident involving an EV:

Use the ERG to find pul fuse location
- Fuses are most commonly used by Hyundai & are usually found:
- In a fuse box under the bonnet
- Behind removable panels in the boot
Pull the fuse upwards
- It WILL NOT detach from the housing, so care must be taken not to accidentally push it back into place

HV will isolate after a Manual Service Disconnect (also known as a Service Plug) is pulled.

The MSD is a HIGH VOLTAGE plug that will isolate high voltage systems when pulled:

It is a direct connection to high voltage systems, including the battery
It is designed for use by trained EV technicians in a controlled environment, such as a workshop, where lock out/tag out procedures can be adhered to
Some ERGs list the MSD as a method of isolating HV systems, however:

EV FireSafe DOES NOT recommend the use of MSD by responders in an emergency situation as responder safety cannot be guaranteed!

Watch: How to isolate an EV battery for emergency responders

04.10 Suppression methods