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Rob Spiegel
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Re: I2R means heat
Rob Spiegel   8/16/2012 4:39:32 PM
Hey, Mblazer, you may want to write up your story and send it along for inclusion in the Sherlock Ohms blog. We need at least 350 words. Simply explain the problem you faced and show how you solved it. 

You can send it to: rob.spiegel@ubm.com

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Re: I2R means heat
Tool_maker   8/16/2012 4:42:55 PM
  I am sorry if this comes of as a smart alec, but did you slap the guy who replaced blown 45A fuses with 60A? There are reasons for the sizes of fuses in any given circuitry and to just ignore that is a recipe for big trouble.

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Re: I2R means heat
mblazer   8/16/2012 6:30:12 PM
Yes we did (verablly at least).  Fortuantely this was just a disconnect box.  The main breaker was was located in another room with 50A breakers.

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Those pesky loose wires
ishkabbible   8/16/2012 9:32:28 PM
I had a similar experience while I was maintaining a large VAX / PDP-11 installation. Every Tuesday morning we would come in and one of the PDP-11/70s would be down. I puzzled over this for weeks, left a program running overnight that just wrote the time to one of the terminals every 5 seconds, and found that it was strikingly regularly crashing at 6:30AM plus or minus a few minutes. The next Tuesday I came in early and was sitting in the computer room at 6:30 - I heard a crash from outside and the front panel lights froze. I ran outside to see the garbage truck pulling away. The dumpster was about 10 feet from the main power box feeding the building. I opened up the box and could see signs of heat on one leg of the 3 phase power. The screw lug holding the wire was loose, and when the garbage truck would drop the dumpster, there was enough vibration to cause just enough of a power glitch to crash the one computer that we had plugged into that leg. Tightened the screw and no more crashes.

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Re: Another loose connection
ab3a   8/17/2012 12:26:15 PM
We have used Infrared imaging since the mid 1980s at least to find and fix these problems before they get ugly.


It ought to be a regular part of scheduled maintenance to pull off the cover of a panel and take infrared pictures of all the screw terminals. 

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Re: Another loose connection
monster69   8/18/2012 1:01:16 PM
1 saves
In my 50th year as a volunteer fireman, I wholeheartedly agree.  Mhy story was my daughters 1st birthday party, also the 1st anniversary of our new house.  There was a pop and the the stove went dead with the roast uncooked.  I checked the breaker and it hadn't tripped.  I turned off the breaker and pulled the stove out, removed the terminal cover since it was hardwired and turned the breaker back on.  One side was open at the stove.  Turned the breaker back off and pulled the panel cover.  Lo and behold the screws on the wire were never tightened.  Properly tightened and everything back together in a half hoiur, the roast was cooked.

William K.
User Rank
Blowing fuses and tripping overloads
William K.   8/18/2012 8:53:28 PM
At one job, as the electrical engineer, I was asked to check into why one of the hydraulic pumps was usually "overloading". At this company it was standard procedure to use an ammeter clamped on one leg of a pump motor to set the relief valve, or adjust the pump compensator. This pump was one of two pumps arranged to assure that the big pumps always had a positive pressure at their inlets. They figured that the pump with the problem must have an excessive load, but the ammeter reading indicated less current on one phase. 

The solution was to tighten the connection terminal screws in the motor starter. One loose screw was causing much more heat to be produced in the connection, causing the thermal overload device to shut down. Raising the trip current setting had just delayed the shutoff time. So with all connections tightened, I reset the overload trip point to the recommended value. The problem was solved and my reputation was enhanced. 

Bill R.
User Rank
Re: Another loose connection
Bill R.   4/19/2013 1:06:56 PM
I agree with the comment you made regarding the essential need for regularly scheduled maintenence of electrical distribution and utilization systems in facilities including infrared scans of all terminations.

To expand upon your recommendation, this work must be done when the electrical distribution system equipment (including but not limited to switchgear, switchboards, transformers, distribution panels, motor control centers, equipment or process control system cabinets or enclosures) have "dead front" (defined as intentionally grounded and not intended to conduct electricity under normal operating and energized conditions) covers have been removed as you indicated.

Removal of the dead front covers while any equipment is energized is an inherently dangerous and possibly lethal activity for the person(s) engaged in the operation.

Work is being done based on IEEE Standards which are being continuously refined, in the industry and and is addressed by OSHA to provide means to determine by calculation of the Arc-flash hazard for new and existing electrical distribution and utilizations systems in new and installations. The Arc-flash hazard classes address the different ranges energy that may exist due to the occurrance of an arc-flash at a specific location in an electrical distribution or utilization system.  Arc-flash energy above Class 4 has no equivalent PPE because anyone within normal working distances from the arc-flash source cannot be protected by apparel.  The blast pressure generated by the arc-flash having an energy greater than the upper limit specied for PPE Class 4 would probably be lethal. 

The point I want to make is that whenever energized electrical equipment is opened, a serious hazard exists.  There are four classes of Personal Protective Equipment (PPE Class 1-4) that have been defined for personnel to wear whenever they are expsed to a specific energy hazard level.  Apparel specific to meet the requirements of each class of protection is available o allow workers to be in specified distances from energized equipment.

I repectfully urge anyone who is involved with the operation, maintenance, construction or modification of electrical distribution or utilization systems and equipment to determine the hazard classes for each electrical system with which you are involved before any proceeding with any operation involving exposed electrical terminations or equipment, especially those activities involving determining whether the equipment is energized or not, and making voltage, current or other electrical state (power, power factor, power quality, etc.) masurments.

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