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Urban mobility, EV vehicles. Dealing with damaged high voltage systems.

Before watching the film take the electricity quiz.

Watch this film then answer the questions. The film is transcribed below.

Since the late 2000's we might have had 100 or so electric vehicles on the road and now the number is 100's and it is growing. So as our prevalence to arrive at a scene and find an electric vehicle increases so do the myths and the realities, the urban legends and the truths. So another thing that we will do in this programme is address some of the misinformation about electric vehicles. There are risks; there are hazards and all that needs to be managed. So no, you wont be electrocuted when you step into the water to work at an incident involving an electric vehicle, and no, you wont be electrocuted when you take your power rescue tool and try to pry a door open. We will take care of those urban legends right off the bat.

How to get a shock

Injury from electricity occurs when we place our body in the path or become a conductor to complete a circuit. DC currents are usually found in devices that are powered by batteries. In order for power to flow there must be a continuous connection or circuit from the negative to the positive side. In this example the electric circuit conducts current from the battery to a motor and powers the motor, then flows back to the battery. If that circuit is broken, for instance by a switch in the path, the current will no longer flow. Therefore, touching one side of the circuit will not result in an electric shock because there would be no completed path for the flow of current through your body. If you touch both sides completing the circuit, however, you could be in for a shock. To receive a shock you must make contact with both hot and negative sides of the circuit. Lack of earth or chassis ground connection means that simply touching one side of the circuit you will not receive a shock.

The inverter/converter

The important thing for rescuers to understand is that the high voltage power is present in the inverter/converter unit and this unit should never be cut or crushed. Now, lets take things a bit further. Inside the inverter/converter unit we will also find high voltage capacitators. Capacitators store and release energy. And in electric vehicles they are used for smoothing out the power travelling from the motor. But for rescuers capacitators could present a serious hazard of shock because they can release up to 400 volts in an instant

The 12 volt battery

All electric plug-ins have a low-voltage system. That would be the 12 volt; the 12 volt that is grounded to the vehicle, to the structure of the vehicle. And that is what we are used to. That is what run up against with internal combustion vehicles as well. In addition to the 12 volt low-voltage system the electric plug-ins will have the high-voltage. Because they have two electrical systems they also have to have at least two batteries. One would be the 12 volt battery and one or multiple batteries would be the high-voltage.

The converter

Lets take a look at the DC to DC converter. This is the alternator for electric vehicles. EVs dont have a motor that can turn an alternator and recharge the 12 volt battery, so they use the DC to DC converter. It is pretty simple, here is how it works. High-voltage current travels from the battery into the DC to DC converter. There it is converted into 12 volt electricity, which then travels to the typical 12 volt battery like we find in most vehicles. It recharges that battery and acts as a second 12 volt system for the vehicle.


Now theres a teaching point about this that I want to make sure you are aware of, related to electric and related to hybrid vehicles as well. All orange cabling you find on an electric and all orange cabling you find on a hybrid vehicle are in fact high-voltagemore than 60 DC; more than 30 AC. However, its not all high-voltage cables are orange. So how do you get around that? Here is how I teach. If you are at an incident and you find a coloured cable, and I am not talking a piece of speaker wire, Im talking jumper cables. If you find something large and it is of a colour, whether it is orange or blue or yellow let your officer know. Get together on it and decide. It may in fact be your first indication that you are dealing with an electric vehicle. Be on the lookout for coloured cables they are trying to tell you something.

How to cut the power in an emergency

Shutting down the high voltage and isolating it to the battery pack is absolutely critical if we are going to be doing any cutting or crushing on the vehicle. Of course we can never shut down the high-voltage to the battery itself, but we can isolate the voltage to the battery by opening up the contacts. Theres a few ways we can do it. Most vehicles are equipped with two different types of disconnect. Safety disconnects and service disconnects. As rescuers we want to use the safety disconnect and try to avoid using the service disconnect. A little bit more on service disconnect in a second. Safety disconnects are built in to EVs for first responders. They can often be found under the hood of the vehicle and occasionally we will find them in the trunk or luggage compartment of a vehicle. They are generally marked very well for rescuers to find. Here we have a safety disconnect marking on the Tesla model S under the front, which would be the hood, but they call the frunk area of the vehicle and it is marked here for rescuers. You can see the firefighter helmet and the cutters showing us that there is an operation that they want us to do under the hood. With the panel removed we can take a look at what they would like us to do. Here we have the label wrapped around the wires that we need to cut. It is two little tiny wires, they are 12 volt wires. And by just simply cutting these wires we can shut down the high-voltage. What it does is it takes the 12 volt power that was being sent to the contacts in the high-voltage battery and it takes that power away, thus opening the contacts and isolating the high-voltage to the battery. If we are unable to access the cut loop or the safety disconnect, we can actually cut the 12 volt battery. We are going to want to cut the 12 volt battery anyway to shut down the SRS system and to just immobilize the ignition of the vehicle. Battery locations vary. We will find them under the hood. We find them under the seats of vehicles. We find them in the luggage compartments. After we locate the battery we always want to cut the negative terminal first and then the positive terminal. We also want to remember to double cut both of our terminals because the memory in the cabling can bring them back together after we have cut them. Many electric vehicles have not only a safety disconnect, but they also have what we call a service disconnect. Generally service disconnects are actually located on the battery itself and this is a high-voltage connection. So when we pull a service disconnect we are actually opening the contacts in the relay. Generally manufactures recommend that only service personal use the service disconnect. Now some manufactures recommend that rescuers use them.

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