Categories: Engineering

Are All Pieces of Your Equipment Grounded Properly? Electrocution on a Die-Cutting Press Due to Poor Grounding

TASA ID: 1006

What Does "Safety First" Mean?

Too often the idea of "Safety First" is a mindset that Management tries to imbue in the Workforce (the hourly production and maintenance workers).  The goal is to have the factory jobs done with no injuries.  This means following the lockout/tagout procedures, using the correct tools for each job, testing the electricians' gloves for integrity, etc.

As managers, we also need to apply the safety idea in our everyday tasks.  These include purchasing (buy materials and equipment that are inherently safe), shipping (ensure that the material or equipment that is produced is packaged in a manner that will not cause an accident), engineering (design and install equipment that conforms to all of the codes), to name a few.  All of these tasks require unique safety considerations.

The NFPA codes cover virtually every area of industrial activity.  One of the greatest reasons that the NFPA came into the forefront of safety codes was in response to the Triangle Shirtwaist fire of March 25, 1911, where 146 women workers died.  In the author's opinion, most of the NFPA codes have been formulated in response to industrial and commercial accidents such as that one.  The one code that most people are familiar with is NFPA 70, the National Electrical Code.  NFPA 70 is perhaps the most basic and ubiquitous of all the NFPA codes.  This case is a presentation of an incident that happened because NFPA 70 was not followed.  Had the equipment grounding requirements of this code been followed, the maintenance worker's life would have been spared.

An Important Lesson to Learn

All pieces of operating equipment must be effectively grounded.  A poor ground is just as bad as no ground at all.

Accident Synopsis

This accident happened during a routine maintenance operation at a raw material supplier's small factory.  The deceased was replacing the dies on a die-cutting machine, called a "Press" in this case.  The press was designed to cut material into the proper shapes from the incoming coil-stock material for further processing.  The worker reached into the press below the die area for a dropped part with his left hand when his right calf came into contact with the adjacent exit conveyor frame.  There was a source of voltage that had energized the conveyor frame and caused his death.

Accident Analysis

The factory electrical supply feed from the power company was from an incoming 480 volt three phase system.  The supply system had a ground fault protective feature on the incoming lines.  The press and the conveyor were fed from a 480 volt to 240 volt three phase step-down transformer.  This transformer had delta phase windings for both the primary and the secondary.  One phase of the secondary was grounded to the building steel and a grounding pad.

When the three phase fused disconnect switch at the press was tested during the post-accident inspection, the center phase read 0.4 volts to ground and the other two read 240 volts to ground.  The building steel framework adjacent to the switch was used as the ground reference for the measurement.  At the post-accident inspection, no electrical fault was found with reference to the press itself.

The 240 volt, 3 phase grounded delta feed to the press was reported to have been shut off when the accident occurred.  With power off to the press, no voltage to ground was found anywhere on the press.  Also, the ohms to ground (again the building steel) from the press measured to be 0.075 ohms.

Prior to the accident, the conveyor was grounded by being connected to a driven "ground rod" located at the exit end of the conveyor.  After the accident, and before the post-accident inspection, the maintenance personnel had run a #8 AWG bare stranded wire from the frame of the conveyor to the base of the press.  This gave the conveyor a good ground through the press and thus to the building steel. Measurements taken with this added ground wire disconnected showed that the "ground rod" had about 90 ohms to the building steel framework.  Thus, at the time of the accident, the conveyor had not been effectively grounded.

If the conveyor had been energized at 120 volts (i.e. from a frayed wire) with a ground as poor as what it was measured it to be, the current drain would have been small.  Using I = V/R, at 90 ohms ground resistance, the current would have been about 1.33 amps.  This would not trip a 15 amp circuit breaker; thus, the conveyor frame would not have been de-energized automatically. (Note that if there had been a 240 volt connection, there would still not have been enough current drain to trip a circuit breaker or blow a fuse.)  Other "clean-up" work had been done before the post-accident inspection by the maintenance personnel.  No evidence of a frayed wire, etc., was found during the inspection.

The main method of grounding the press was through the green wire of a 4-wire flexible cable that fed into the electrical box on top of the press.  This ground wire was there at the time of the accident.  After the accident, the maintenance man had installed a ground to the inside of the disconnect switch for the press.  Both of these ground wires got their grounding from being bolted to the inside of the junction box in the truss area where the flex cable was connected to feed the press. Thus they are tied electrically to the building steel, as was one leg of the 240 volt delta transformer secondary.

At the post-accident inspection, no evidence of leakage voltage to either the conveyor or the press was found.  The press switch was reportedly in the "Off" position, and the conveyor was reportedly not running when the accident occurred.

Accident Cause

At the time of the accident, the press was solidly and effectively grounded and the exit conveyor was not, since it was connected to a ground rod that had high resistance to the building and the electrical system ground. The exit conveyor must have been energized by some electrical connection through faulty wiring of some nature.  As noted above, since the resistance to ground was as high as it was, the ground current was not high enough to trip a normal circuit breaker that was rated at 15 amps.  There were no "ground fault interrupter" type circuit breakers on the 240 volt side of the main step-down transformer.

The major contributor to the accident was the improper grounding of the conveyor frame.  Had it been properly grounded, then whatever caused the frame to be energized would have created a short that would have tripped the supply circuit protective device, thus removing the dangerous condition.

Using a driven ground rod instead of connecting the equipment to the building framework for the conveyor ground was the basic mistake made.  All grounding systems must be effectively bonded together in order to achieve the electrical safety that the code requires.  NFPA 70, The National Electrical Code, Article 250 states in Section VI. "Equipment Grounding and Equipment Grounding Conductors":"250.110 Equipment Fastened in Place or Connected by Permanent Wiring Methods (Fixed). Exposed noncurrent-carrying metal parts of fixed equipment likely to become energized shall be connected to the equipment grounding conductor..."

In this case that "equipment grounding conductor" for the conveyor should have been the frame of the press (as was done after the fact), the green wire in the four conductor cable that fed the press, or the building steel.

Here is a case where the basic equipment engineering design and installation negated any "Safety First" programs that Management could have instituted for the Workforce.  Management also needs to be cognizant of their requirement to apply all Safety First ideals to its activities.

This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances.  Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.  

This article may not be duplicated, altered, distributed, saved, incorporated into another document or website, or otherwise modified without the permission of TASA.
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