Marshall Pruett's Racing Tech Mailbag for March 7

Marshall Pruett's Racing Tech Mailbag for March 7S

Welcome to Marshall Pruett’s new Racing Tech Mailbag on We had so may questions for the first edition that we had to run this in two parts.

If you have questions about the technical side of the sport for Pruett, who spent most of his life working as a mechanic and engineer in open-wheel and sports car racing, send them to We can’t guarantee your letter will be published, but Marshall will always reply.

I’m curious about the safety aspect of the electric hybrids. With high amperage batteries, and super capacitors, is there a risk of shock for corner workers & drivers (in a crash that heavily damages the car)? Also, what are they doing to minimize the risk to mechanics? I haven’t heard of any problems, but I hope that continues to be the case.

Randy Word

MP: It’s a tricky subject, Randy. Hybrid P1 cars are required to have a simple green or red light next to the external kills switch for workers (and anyone else) to monitor whether the system has been discharged and the car is safe to touch. But what if that part of the car is damaged in a crash? There’s also a green/red light system in the cockpit, so that’s a backup, but in the event of a “big one,” you’re always going to be concerned about being the conduit for anywhere between two and eight Megajoules of energy.

Corner and workers and safety personnel are trained on dealing with Hybrid cars, just like they are with the non-Hybrids, so that’s a plus, and Hybrid teams have hand-held monitors they use to check the discharge when the cars are stationary.

Thisinformationcan also be acquired through telemetry when the cars are on track.
I reached out to my man Owen Hayes, Porsche Motorsport North America’s director of operations who oversaw the manufacturer’s 911 GT3 Hybrid program (that Hybrid knowledge went into the new 919 P1 car), and asked him to go describe their safety measures in greater detail:

The subject of protecting people around hybrid cars is a subject that is obviously taken very seriously by Porsche. I have also attached a “fire brigade and track marshal’s instruction sheet” [click here to view by PDF] which we distributed in the past to brief track personnel.

Working on the car:If someone is working on the car, the hybrid system is fully discharged and that’s why you see the red and green signs on top of the cars. Red means the hybrid system is still charged. Green means the system is fully discharged. However in case of someone inadvertently opening a high voltage connector, the high voltage cabling has an electrical protection circuit which monitors and detects the opening and closing of the high voltage connections. If a connector is opened when the system is charged, this is detected by the control unit and the hybrid control unit opens a main relay in the inverter of the energy storage, immediately cutting off high voltage to the vehicle wiring.

Marshall Pruett's Racing Tech Mailbag for March 7S

There is also what we call an insulation monitor fitted to the car, the function of which is to constantly diagnose the high voltage insulation/shielding. If an error occurs, this is detected by an “isolation switch” and the main relay in the inverter is opened, thereby immediately cutting off the high voltage to the vehicle wiring.

Visual indication:With the GT3 R hybrid for example, the hybrid system operating mode was indicated by red lamps at the front and rear window. When the hybrid system was active, both the front and rear lamps were switched to red on at a voltage above 50 volts. As soon as the hybrid system was switched off, the red lamps would fade out when the voltage went below 50 volts. There were also LEDS fitted to the voltage inverter, so once again if the voltage is above 50 volts, the lamp was red, below 50 volts, the lamp was green. Below 50 volts is not harmful to us humans!

Crash situation:If a crash is detected via an acceleration sensor, the hybrid control unit will immediately shut down the hybrid system and cut off the high voltage to the vehicle wiring.

Emergency off switch:If the car stops on track and the hybrid system has to be discharged, the outer emergency switch can be pulled which shuts down the entire vehicle and hybrid system. In addition, the fire extinguisher is activated.

(Continued with more than a dozen other Q&A items at

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Written by Lewis Shaw

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