Red lion G308 blown up during programming

ggc

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Join Date
Apr 2007
Location
Queensland
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In the last 2 weeks I have managed to blow up 2 *Red lion G308 screens during commissioning of a modification to an existing machine. The problem appears to be a potential difference across the ports of the laptop, I'll lay out the scenario here and see if anyone has any further suggestions before we blow up another one.

Destruction 1: Existing G308 with opto-isolated serial ports.
New ethernet cable was run from the G308 to a network switch, then onto a VIPA 313 PLC. Cable was run in close proximity to resistance welding SCRs (thyristors). Panasonic toughbook was connected via USB to the G308, via a serial port and PM2 cable to an Allen Bradley Micrologix 1500, and via cat 5 cable to the network switch. The laptop was also connected to mains power, which is not fed from the same supply as the machine. All items inside the cabinet are tied back to the same earth, and are all supplied from the same 24V DC supply. On energisation of the resistance welding SCRS, the 24 V supply was dragged down until it eventually tripped, the G308 now has a black screen, no comms and the smell of failure, and all USB ports on the laptop are fried. Conclusion was that the SCRS had induced a potential difference across the new ethernet cable, which eventually resulted in significant current being pulled down the USB programming cable. Ethernet cable was rerouted outside of the cabinet and a new G308 screen installed. Earths were checked, no potential difference between any of the items in the cabinet was noted.
Destruction 2:
Machine had been happily running for a few hours after recommissioning. IBM laptop plugged into mains power, USB port 1 connected to red lion using quality USB cable, cat 5 cable connected to ethernet switch. Operator requested program change in the AB Micro 1500, PM2 cable was connected to a USB-serial converter, on plugging the USB cable into the laptop, a small spark was noticed. A few seconds later, the stench of failure was noted coming from the red lion. USB cable has extensive burn marks on the ground pin, and the USB port on the laptop is fried. Earths were checked again, everything in place.

Can anyone suggest what has happened and why?
 
Cannot suggest a reason but have seen it before - cost me a new motherboard and a trip to the states for the screen. Mine was the outdoor one and it also blew up the USB port in the laptop.
The laptop copped the blame of course but it had been happily programming Omron PLCs and screens for some years through the exact same USB port.
Have not used one since by the way but will have to shortly as I will need outdoor screens again.
The screen was mounted in the control cabin for one of the Sydney Harbour Bridge cranes.
Great view by the way, got sick and tired of bridge climbs though with a laptop on the back and a toolbag in hand. It is quite a climb for an old fella!
Smokers legs and lungs kick in too of course but the view is breath taking - highly recommended to all in Ozz and also all overseas visitors.
Take 2 climbs, one during the day and one at night - I have done both of course but it is no fun at 2AM when you are half asleep.
 
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My first pick would be to check that the two leads from the welder to the welding electrodes are in good condition,no breaks, no insulation damage, and not to earth except at the welding tip.

To me it appears that the mains Earths, are providing a better path for current than the welding leads, this optional path probably should not exist.
 
Indirectly, it says that we are talking about different locations of the red lion HMI, the ethernet switch, and the VIPA PLC, is that right ?
In addition a laptop was connected which is probably supplied from a different power source, right ?
My guess is that a good common equipotential earth is missing.

Earths were checked, no potential difference between any of the items in the cabinet was noted
[...]
Earths were checked again, everything in place
It is not good enough to check the earth inside a panel.
Check the potential difference between locations.
Check the potential difference of the power source used for the laptop to the panel chassis.

And, the galvanic isolation of ports will only help in a minor degree to save the directly connected devices. But it does help to limit the catastrophe to only the directly connected device.

*: We normally provide an outlet that is powered from the panel for a programming laptop. For exactly this reason.
 
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I'll jump and provide this theory:

The problem is a poor earth connection for the outlet that is used for the laptop. This outlet shares the earth with the welding device. When the welding is active, the current has to return through common earth (should not, but does (*)), which again increases the voltage potential of this earth connection. This generates a voltage difference between panel earth and outlet earth.

*: Also investigate if there is a proper return path for the welding current.
 
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I'm definately leaning towards the earth on the laptop supply being a source of the problem. I've been working on this machine many times, but never with the laptop power supply as it's only ever been for short periods.

The fact that it's a welding machine may be a red herring, both times the fault occurred the SCRs were supplied power, however they were not welding when things went bad.
 
I didnt say that the laptop power supply by itself was the problem, i.e. that the power supply has a defect.
I mean that there are voltage differences between the various earths in the plant.
Simply measure the voltage between the erath of the laptop supply and the panel supply. Let the meter hang for a long time and let it hold the max value.
 
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My general suggestion to most customers is to run your laptop on battery power if at all possible. This eliminates a lot of grounding problems caused by the laptops poor or non existent ground. Most likely you did experience a ground potential, which would cause what you have described.
 

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