HV potential equalization line

Balderdash999 · Aug 20, 2020

Hi all,

Hopefully a quick one, I feel I may have missed something out of my calculations here.

In HV vehicles all HV components will be bonded to the vehicle body, typically via its own independent bonding strap which has a low resistance. I understand that this is to provide an alternative route for the current under a fault condition i.e. most of the current will flow through this ground strap rather than the human.

My question is.. Say the vehicle has a 300V supply, and the resistance of the PE line is 100ohms and the typical human resistance is 2000ohms, under a fault condition you will see;

PE line - 300/100 = 3A?
Human - 300/2000 = 150mA?

At first it looks right, as the majority of the current is going through the ground strap, however I cant understand why there is still 150mA that would be going through the human, as this is still very high and can still be fatal. I feel there is slightly more to my calculation than a simple 2 resistances in parallel.

Can someone please point me in the right direction.

rjenkinsgb · Aug 20, 2020

The earth system resistance would be more like in the milliohms range than 100 ohms. Earth cables are typically the same size as the power cables, but likely also with multiple alternate current paths.

The current should cause a shutdown or blow a fuse within milliseconds.

Also, human dry skin resistance is more like 100K - 5M, and the person would need to simultaneously be touching two different parts of the metalwork before there would be any possibility of current flow through their body.

We are presently part way through a rewire on a rather large electric vehicle with a 500V system. The earthing consists both of direct chassis bonding and a 50mm^2 cable along each side of the vehicle, with additional chassis bonding from that.

50mm cable has a resistance of approximately 0.38 Ohms per kilometer... The end-to-end earth resistance of the vehicle wiring will be a couple of milliohms, with the chassis metalwork in parallel with that.

Nigel Goodwin · Aug 20, 2020

Balderdash999 said:
In HV vehicles all HV components will be bonded to the vehicle body, typically via its own independent bonding strap which has a low resistance. I understand that this is to provide an alternative route for the current under a fault condition i.e. most of the current will flow through this ground strap rather than the human.

Why are you even assuming that a fault condition would cause current to flow through a human?.

By FAR the most likely reason would be a human doing something utterly stupid - Darwinism at work.

It would be difficult to envision a 'fault' which would somehow magically fry the occupants?.

rjenkinsgb · Aug 20, 2020

Nigel Goodwin said:
Why are you even assuming that a fault condition would cause current to flow through a human?.

As that's what the OP was comparing:

Balderdash999 said:
I understand that this is to provide an alternative route for the current under a fault condition i.e. most of the current will flow through this ground strap rather than the human.

My question is.. Say the vehicle has a 300V supply, and the resistance of the PE line is 100ohms and the typical human resistance is 2000ohms, under a fault condition you will see;

PE line - 300/100 = 3A?
Human - 300/2000 = 150mA?

Balderdash999 · Aug 20, 2020

rjenkinsgb said:
The earth system resistance would be more like in the milliohms range than 100 ohms. Earth cables are typically the same size as the power cables, but likely also with multiple alternate current paths.

The current should cause a shutdown or blow a fuse within milliseconds.

Also, human dry skin resistance is more like 100K - 5M, and the person would need to simultaneously be touching two different parts of the metalwork before there would be any possibility of current flow through their body.

We are presently part way through a rewire on a rather large electric vehicle with a 500V system. The earthing consists both of direct chassis bonding and a 50mm^2 cable along each side of the vehicle, with additional chassis bonding from that.

50mm cable has a resistance of approximately 0.38 Ohms per kilometer... The end-to-end earth resistance of the vehicle wiring will be a couple of milliohms, with the chassis metalwork in parallel with that.

Yes i agree with what you're saying but the sole purpose of the PE line on HV components is to provide an alternative route for the current to flow if somehow a human manages to short themselves between HV - and HV + on 2 components.

To answer Nigel's question, If for example number 1 HV component has a HV+ to ground short and HV component number 2 has a HV- to ground short and a human was to come along and touch both of these components, they would be electrocuted. This is the purpose of the PE line, so in theory under the above fault conditions, a human can still touch both components but because their body resistance is higher than the resistance between the 2 HV components, they will only get minimal current... because the bulk of the current will go through the PE line/vehicle body.

If we use the figures rjenkinsgb used, 500V supply, and more realistic resistance for the HV component ground straps of 200mohms, the human body resistance can be as low as 1000ohms given the right conditions (so lets work with a worst case scenario).

So under a fault condition;
current through the PE line = 500/200mohms = 2500A
current through the Human body resistance = 500/1000 = 500mA

So the current theory looks correct, i.e. higher amounts of current will pass through the vehicle body, but 500mA through a human is still fatal, so in principle the PE line is pointless? And this is where i suspect my above calculations are missing something? or am i not?

Nigel Goodwin · Aug 20, 2020

Balderdash999 said:
Yes i agree with what you're saying but the sole purpose of the PE line on HV components is to provide an alternative route for the current to flow if somehow a human manages to short themselves between HV - and HV + on 2 components.

To answer Nigel's question, If for example number 1 HV component has a HV+ to ground short and HV component number 2 has a HV- to ground short and a human was to come along and touch both of these components, they would be electrocuted. This is the purpose of the PE line, so in theory under the above fault conditions, a human can still touch both components but because their body resistance is higher than the resistance between the 2 HV components, they will only get minimal current... because the bulk of the current will go through the PE line/vehicle body.

If we use the figures rjenkinsgb used, 500V supply, and more realistic resistance for the HV component ground straps of 200mohms, the human body resistance can be as low as 1000ohms given the right conditions (so lets work with a worst case scenario).

So under a fault condition;
current through the PE line = 500/200mohms = 2500A
current through the Human body resistance = 500/1000 = 500mA

So the current theory looks correct, i.e. higher amounts of current will pass through the vehicle body, but 500mA through a human is still fatal, so in principle the PE line is pointless? And this is where i suspect my above calculations are missing something? or am i not?

As you've already been told, the resistance of a human body is considerably higher than 1000 ohms - your other misconception is that the battery voltage is 500V - IF the bizarre and unlikely series of faults you envisage occurs, then 2500A drawn from the battery will mean the battery voltage will be a LOT less than 500V (dependent on wiring and the batteries internal resistance). I would also expect drawing 2500A to blow something, presumably there's a huge fuse for just such an occurrence.

JimB · Aug 20, 2020

Balderdash999 said:
but the sole purpose of the PE line on HV components is to provide an alternative route for the current to flow if somehow a human manages to short themselves between HV - and HV + on 2 components.

Wrong.

Balderdash999 said:
If for example number 1 HV component has a HV+ to ground short and HV component number 2 has a HV- to ground short and a human was to come along and touch both of these components, they would be electrocuted

Wrong.

Balderdash999 said:
So under a fault condition;
current through the PE line = 500/200mohms = 2500A
current through the Human body resistance = 500/1000 = 500mA

What about the fuse or circuit breaker in the 500v supply?
That is going to trip out very quickly with this very high overload current.

Also the internal resistance of the supply will cause the output voltage to drop under such overload conditions.

Balderdash999 said:
this is where i suspect my above calculations are missing something?

You are missing any practical understanding of electrical circuits.

Your whole concept and understanding is flawed.

JimB

Balderdash999 · Aug 20, 2020

Hmmm okay! thanks for the feedback. Back to the drawing board

Diver300 · Aug 20, 2020

The high voltage (400 V or so) systems in electric vehicles are all isolated from the body of the car. I think that is universal in the modern electric cars.

The 12 V systems are run through isolated converters. The 12 V systems on the electric cars are very similar to the 12 V systems on internal combustion cars in that the ground is connected to -ve of the 12 V battery.

kubeek · Aug 20, 2020

Also, the impedance between the HV parts and the chassis is monitored, and if it falls below some threshold the HV system is disabled and the main contactors will be off.

HV potential equalization line

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