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bonding case grounding lug to negative current carrying lug.

John Frum

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Nov 30, 2019
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Scenario:
Simple mobile system off grid system.
You have an inverter with a case grounding lug in addition to the current carrying positive and negative terminals.
You wire the inverter to a positive and negative busbars.
The negative busbar is bonded to the frame.
Instead of wiring the case grounding lug to the negative busbar.
Is it ok to just jumper the case grounding lug to to the inverter's own negative terminal?
If not, why not?
I've not seen it done so I suspect there is a good reason not to.

I've got some ideas but don't want to poison the well.
 
My Samlex PST-2000 manual states "Connect the case lug to the vehicle chassis or dedicated DC ground Bus."
It doesn't say to not do this as you ask. But it specifically states the above.
I did read in some manual to not connect the case lug to the negative input terminal, but I can't find that reference.
I suspect that because the inverter case lug is to create an earth ground for the ac voltage, a direct connection to the chassis is prefered.
My electric system logic mind can't see any real reason to not use the negative input terminal, except, why did the manufacturer include a case lug?
 
My Samlex PST-2000 manual states "Connect the case lug to the vehicle chassis or dedicated DC ground Bus."
It doesn't say to not do this as you ask. But it specifically states the above.
I did read in some manual to not connect the case lug to the negative input terminal, but I can't find that reference.
I suspect that because the inverter case lug is to create an earth ground for the ac voltage, a direct connection to the chassis is prefered.
What is the logic behind the preference?
My electric system logic mind can't see any real reason to not use the negative input terminal, except, why did the manufacturer include a case lug?
If the case is internally bonded to the negative terminal then you would not have a choice.
 
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Option, I want A/C chassis ground and D/C negative separate unit chassis connection.
If A/C chassis ground is being used something is going very wrong on the A/C side and I don’t want to risk it affecting D/C devices. Or the potential to remove a electrified ground.
 
I recommend an independent wire to the frame to remain connected independent of servicing or faults in other areas of the electrical system.
If no frame such as a truck camper, go direct to the 120v ground bus in the distribution panel.
 
My Samlex PST-2000 manual states "Connect the case lug to the vehicle chassis or dedicated DC ground Bus."
It doesn't say to not do this as you ask. But it specifically states the above.
I did read in some manual to not connect the case lug to the negative input terminal, but I can't find that reference.
I suspect that because the inverter case lug is to create an earth ground for the ac voltage, a direct connection to the chassis is prefered.
My electric system logic mind can't see any real reason to not use the negative input terminal, except, why did the manufacturer include a case lug?
I had the same question and in fact on my inverter apparently they are all connected anyway. I have a Renogy 1000w inverter and I get continuity between the ground lug, the negative terminal and the AC neutral and ground (which are internally bonded). So on mine that ground lug doesn't need a cable so long as the negative DC cable gets grounded to a chassis ground (via bus bar counts is what I was told by Renogy). All that said, I hear mixed things on the brand so I'm curious if others get continuity between their ground lugs and negative DC terminals already. I'm still a bit skeptical.

Check out that thread:
 
The ground case lug on the inverter is to provide a path for any internal fault in the DC or AC parts of the inverter that would make the casing 'live'. This should be an additional cable connected to vehicle chassis/ body , one size down on the high DC current cables.

Connecting the case lug to the inverter DC negative would not provide an AC path if there were a battery DC disconnect ( BMS, fuse, maintenence, isolator).

Note battery negative should be connected to vehicle 'earth', frame/chassis/metal body.

The protective earth conductor of any AC system should also be connected to vehicle 'earth'.

Any existing measured continuity with the inverter, case/DC negative/AC protective earth, cannot be used as an alternative, as these paths may, fail mechanically, example internal loose connection, or may not be able to safely carry fault current.

Mike
 
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