Rail and chassis potentials during a transient

[kellogs]

Suppose a -100V transient arrives at some place between rail and chassis. Does any of the two points of this place remain at the same potential relative to what each of the points potential was previous to the transient or do they both change? and if so, how ?

Regular operation, Moment t1:
V_in(X,t1) = 13.5V
V_gnd(Y,t1) = 0V

-100V Transient arrives, Peak Moment t2:
V_in(X,t2) - V_in(X,t1) = delta_V_in
V_gnd(Y,t2) - V_gnd(Y,t2) = delta_V_gnd

what would the delta_V_gnd and delta_V_in be ?

 

[crutschow]

Does any of the two points of this place remain at the same potential relative to what each of the points potential was previous to the transient or do they both change? and if so, how ?

All voltage measurements are relative between two points so, when you ask "do they both change", the question is "relative to what"?

 

[Diver300]

On a vehicle, the chassis voltage can be just about anything when measured relative to the ground under the vehicle. It's common to take the chassis voltage as zero, because everything is referenced to that, and it is only voltages between other bits of wiring and the chassis that make a difference to the vehicle components.

 

[kellogs]

All voltage measurements are relative between two points so, when you ask "do they both change", the question is "relative to what"?

And this is why i did not mention "voltage" anywhere.

i am looking for some differences in potential on the same spot, between two moments in time.

 

[Papabravo]

And this is why i did not mention "voltage" anywhere.

i am looking for some differences in potential on the same spot, between two moments in time.

In order to measure ANY kind of potential, you MUST define a reference point. There is simply no way around this aspect of physics. Once you do that it makes sense to talk about the change in potential as a function of time. Short of that you just spewing word salad.

 

[kellogs]

Ok, let me quickly reframe to include Earth everywhere then:

Regular operation, Moment t1:
V_in(X,t1) = 1013.5V
V_gnd(Y,t1) = 1000V

-100V Transient arrives (ISO 7637-2 type of thing, this one is not referenced to Earth), Peak Moment t2:
V_in(X,t2) - V_in(X,t1) = delta_V_in
V_gnd(Y,t2) - V_gnd(Y,t2) = delta_V_gnd

what would the delta_V_gnd and delta_V_in be ?

 

[Diver300]

what would the delta_V_gnd and delta_V_in be ?

They depend on all sorts of factors, such as the capacitance of the vehicle body to earth, the capacitance of the input rail to earth.

In most cases, the capacitance of the input rail to earth will be just about zero, as the input rail will only have capacitance to the vehicle body. In that case, the voltage between the vehicle body will and ground not change significantly during a transient.

However, the voltage of the vehicle body to ground doesn't make any difference to the effect of any voltage within the car. Electronic modules on the car are not connected to the earth, only to the vehicle body, so it just doesn't matter. All that matters to them are the voltages between the terminals of the module.

If I measure the voltage of a vehicle body to ground, the reading will be meaningless and affected by lots of things. It's very common to pick up the ac mains frequency when anywhere near normal mains supplies.

The situation with the 12 V batteries is similar. Most of the time, the -ve is at the same voltage as the car body, becasue there is a great big wire keeping them at the same voltage, and that leaves the +ve terminal at 12 V. However, when that wire is not there, then the voltage on the battery terminals is different. Some common situations are:-

Body disconnected from battery -ve. Battery +ve will normally be at body voltage, battery -ve will be at -12V
The second 12 V battery in a 24 V system, where two 12 V batteries are in series. Second battery -ve is at 12 V, and the second battery +ve is at 24 V.
Positive earth vehicles. Battery +ve is at body voltage, battery -ve is at -12 V.
Battery disconnected both ends. Both voltages unknown and unimportant, but there will be 12 V difference between them.

The point is that all voltages have to be referenced to something, and if there is nothing to reference to, there cannot be a complete circuit, so there can be no electrical effect.

 

[kellogs]

They depend on all sorts of factors, such as the capacitance of the vehicle body to earth, the capacitance of the input rail to earth.

In most cases, the capacitance of the input rail to earth will be just about zero, as the input rail will only have capacitance to the vehicle body. In that case, the voltage between the vehicle body will and ground not change significantly during a transient.

Then delta_V_in =~ -100V and delta_V_gnd =~ 0V. Did I get it right ?

 

[crutschow]

And this is why i did not mention "voltage" anywhere.

i am looking for some differences in potential on the same spot

And how do you think "potential" is measured if not in volts?

 

[Diver300]

Then delta_V_in =~ -100V and delta_V_gnd =~ 0V. Did I get it right ?

Well that is what I would guess. However my guess is based on some assumptions about the physical arrangement, and I know that very small changes in conditions will make a big difference to the voltages and to the voltage changes.