The reason why CAN protection devices have a stand-off voltage above 24 V is not linked to the CAN bus itself. You are right: most of the CAN transceivers work below 5 V.
But in an automotive environment, CAN protection must be compliant with many standards simulating incidents/mistakes that can occur in a car.
In particular, a car must be robust in case of a “jump start” (ISO 16750 standard). This test consists in applying 24 V on the CAN bus to simulate for example:
- A wrong connection of an auxiliary battery in series with a flat battery of a passenger car;
- A garage battery booster with a wrong voltage selection (24 V instead of 12 V) connected to power a passenger car with no battery;
- A truck battery (nominal voltage 24 V) connected to power a passenger car to start the engine.
And certainly, others I cannot imagine.
The test duration is 60s +/- 10%. The normative constraints are described in the doc below:
CAN-bus-protection-ST-ESDCAN-series presentation
A CAN TVS with a 5 V breakdown voltage would enter in avalanche mode with an unlimited current and would burn well before the end of the 60s of test duration.
So the principle for the TVS is to avoid entering conduction mode, and therefore, to increase the breakdown voltage to above 24 V.
That’s a compromise with the clamping voltage, but most of the automotive CAN transceivers have absolute maximum ratings (AMR) above 24 V. However, these transceivers cannot pass ESD and ISO 7637 transients alone without the help of a CAN TVS.
You may see that some of the CAN TVS protection devices have stand-off voltages of up to 35 V or 36 V. These products are intended for commercial vehicles (trucks, off-roads, …) with 24 V batteries, like ESDCAN05 or ESDCAN06 for example.
If your application is not linked to automotive, then you may find TVS with a better compromise as all these automotive standards do not apply. But it depends on your CAN protocol implementation. Indeed, the voltages tolerances are quite wide.
Keep in mind that the TVS in your circuit must be as close as possible to the connector (better to stop the transients at the entrance) and the stand-off voltage selection must be as close as possible to the operating voltage of your CAN bus including the tolerances.
For example, if your CAN bus operates between -0.3 V and 5.5 V maximum, then choose a unidirectional TVS diode with a stand off voltage up to 5.5 V.Do not select a bidirectional 5.5V diode that will clamp negative transient voltages below -5.5 V only!The unidirectional will clamp negative transient voltages below -0.6 or -0.8 V. It is much safer.
I hope this clarifies things.