Sherlock Ohms

Strap Grounds Field Failures

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William K.
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Re: Grounds, shields, and ungrounded grounds
William K.   1/25/2013 5:32:12 PM
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.

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Less than Expert Customers
Tool_maker   1/28/2013 1:02:50 PM
  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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Automotive Ground Straps
Critic   1/28/2013 4:51:55 PM
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
Re: Automotive Ground Straps
bob from maine   2/6/2013 1:42:52 PM
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.

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