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Sherlock Ohms

Strap Grounds Field Failures

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Nancy Golden
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Platinum
Customer Modifications
Nancy Golden   1/23/2013 12:27:49 PM
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That is a very interesting story and great job about troubleshooting a field failure with an uncooperative customer...it is also a great reminder to inquire as to any modifications that a customer may have made that can introduce problems that they are blaming on the original design!

Tim
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Platinum
Re: Customer Modifications
Tim   1/23/2013 7:19:09 PM
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It was good of the vendor to offer a solution to the customer's modification. Good vendors will help as long as safety is not concerned. As a teen, I was working on an old Chevy loosening a bolt that was too close to the frame to reach with a standard box wrench, so I ground some off of the OD which promptly broke. I then took the Craftsman wrench back to Sears who said that it had been modified, but it should have been tough anyway and gave me a replacement.

Charles Murray
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Blogger
Re: Customer Modifications
Charles Murray   1/23/2013 8:07:04 PM
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The Sherlock Ohms stories never fail to amaze me. I never would have considered adding a ground strap between the wagon body and axle frame. Engineers -- especially those who submit our Sherlock Ohms stories -- are an amazingly clever bunch.

Jennifer Campbell
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Gold
Re: Customer Modifications
Jennifer Campbell   1/23/2013 9:05:12 PM
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They sure are, Chuck. I often wish I had the engineering chops to fix my own car, dishwasher, or washing machine after something goes wrong.

This story, about a freight train interrupting a cell phone signal, just might be my favorite.

DanSchwartz
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Iron
Re: Customer Modifications
DanSchwartz   1/24/2013 9:38:06 AM
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@Jennifer: My favorite Sherlock Ohms article so far is Noise Messed With the Automation System; which led to this popular article Surge Impedance: Why you see the spark when you unplug your iron I wrote to explain the underlying engineering principles.

Dan Schwartz,
Editor, The Hearing Blog
All incoming LinkedIn colleague and Facebook friend requests are welcome~

Charles Murray
User Rank
Blogger
Re: Customer Modifications
Charles Murray   1/24/2013 6:53:17 PM
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I remember that story, Jenn. I liked it for two reasons. First, it was a clever solution. Second, instead of coming up with some costly, convulted solution, they simply stopped testing their phones when a frieght train would pass.

dougspair
User Rank
Gold
Re: Customer Modifications
dougspair   1/24/2013 5:00:57 PM
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 On the subject of ground straps, I've seen missing ground straps on motor vehicles, and what often happens is the cranking current for the starter motor will go through the next best conductor, often the driveshaft U-Joints, fries them pretty well.

Critic
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Platinum
Automotive Ground Straps
Critic   1/28/2013 4:51:55 PM
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Instructions for jump-starting vehicles typically recommend that the last jumper cable connection be made to the chassis of the vehicle, rather than to the battery itself.  This is supposed to prevent a spark near the battery vents where hydrogen gas could ignite.  This is not bad advice to prevent a hydrogen explosion, but there is generally very little risk of an explosion (safety conscious people:  bash me now).

If the dead/low battery is no longer capable of sourcing enough current to crank the engine after a few minutes of charging, then a different kind of fire hazard is created by following the "safe" instructions.  The heavy-gauge negative cable from the battery typically connects to the engine block, because the starter usually draws more current than any other electrical device in most (production, non-electric, non-hybrid) cars and trucks.  There is usually a lighter-gauge strap from the engine to body/chassis.  If you connect the jumper cable to the chassis, and the cranking current must flow through the jumper cable, that is, if it is not sourced by the dead/low battery, then cranking current flows through the strap.

In this situation, the strap gets very hot, very quickly.  I witnessed a not-so-helpful tow-truck driver try to jump start my truck and the procedure melted the ground strap and started a small fire.

One reason this doesn't happen more often is that the jumper cables most people own are of small wire gauge, so there is significant voltage drop across the cable during cranking.

Does anyone know of a safe, non-destructive procedure for jump starting???

bob from maine
User Rank
Platinum
Re: Automotive Ground Straps
bob from maine   2/6/2013 1:42:52 PM
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The comment about small ground straps from the body to the engine block is a valid point. If you connect the negative cable to the engine block - not the frame -  you will be fine. Making a good electrical contact with a jumper cable clamp and a rusted or painted part of a steel engine block may be difficult so you may have to choose the connection point carefully. Providing an adequate ground on moving equipment has proven to be a diffuclt challenge to meet. Imagine the static that used to build-up on cars in the 50's. Eventually methods were developed to allow the static charge to bleed-off, but before then, passing a bill to a toll attendant could cause some fairly dramatic shocks.

eafpres
User Rank
Gold
Customer Service
eafpres   1/24/2013 2:11:22 PM
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Hi Chip--great story.  It is always challenging supporting customers in the field.  You were lucky to get the parts back.

Once I was involved with design, production, and then support of an antenna module for commercial telematics.  It consisted of an amplified GPS patch antenna integrated to a cellular omnidirectional antenna printed on an FR4 PCB.  There was a long cable pigtail to allow the installer to locate the antenna in a good but hidden place in vehicles.  Most often, it was placed under the dash on top of wire bundles or air ducts etc.

Two support challenges were memorable.  I don't need to give all the details, just the solutions.  Case 1: "We recommend you do not locate the antenna inside the glove box as the antenna won't perform well inside of a metal enclosure".  Case 2: We added a label "This Side Up" to ensure the GPS patch was facing the sky, not the ground.

William K.
User Rank
Platinum
Grounds, shields, and ungrounded grounds
William K.   1/24/2013 7:15:35 PM
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This is an interesting example of one of the problems that will pop up when the efforts to keep low level signals in the low level realm are inadequate. I can understand that it was cheaper to simply glue the strain gauge to the axle, but the shiels of the cable should have been connected to that axle, or another conductor, if not the shield. MY reasoning is that aside from the voltage build up, the conductive axle is a good candidate to pick up a lot of noise and couple it to the strain guage. I learned all about the coupling of undesired signals while working with a non-strain-gage pickup as part of a product development project. Compensating cor the mount material potential is vital. 

The two solution options would have been to either ground the axle to the input circuit, or to float the input circuit from all other potentials, but that would be difficult and expensive.

tekochip
User Rank
Platinum
Re: Grounds, shields, and ungrounded grounds
tekochip   1/25/2013 8:42:19 AM
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Good point William, in practice the load cells were never attached to the shield because of problems with ground loops.  Some of the load cells were trailer hitches or physical links between the wagon body and wagon frame.  Tail lights, alarm horns and other accessories would wreak havoc with the signal if the shields were terminated.

William K.
User Rank
Platinum
Re: Grounds, shields, and ungrounded grounds
William K.   1/25/2013 5:32:12 PM
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Wit our sensors, which detected shock waves traveling in the metal, I found that every installation needed to either pick up the substrate that the sensor was mounted on, or else have a very thin insulkated shield bonded to the substrate, with the sensor then bonded to the shield. This avoided there ever being more than a few volts between the sensor and the conductive surface that it was mounted on. That might be a problem for strain gauges, I have not studied it. Possibly a rigid enoug epoxy would work.

Tool_maker
User Rank
Platinum
Less than Expert Customers
Tool_maker   1/28/2013 1:02:50 PM
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  I just love it when customers or end users make changes, without understanding the logic of what they have changed, and then blame the designer for producing a defective part. This problem runs across the full spectrum and is not confined to weak minded people. If two is good, why three must be better. I am sure this vehicle would perform better with a different tire size. Etc, etc.

  Great job of trouble shooting and you did not lose a customer, you solved a problem yet to come.

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