Problem Solving Article: Electrical open ground on receptacle wiring 

An Article from the Series of Problem Solving Articles in an Industrial Setting

I worked for a large cannery in Modesto, CA, as an Instrumentation/Electrical Technician. We had four lines for filling plastic, snack food cups with fruit and syrup or fruit and gel.

The continuous Eagle lines had a knife cutter that simultaneously cut around the cap of six cups at once, on the fly. Each cutter had a single phase, 277VAC heater to maintain a temperature of around 360 degrees F. on the metal knifes. Each Eagle line also had 16 additional heaters with the same connection setup.

The power feed from a stationary connection box was through the Woodhead Safety Yellow, three pole, flexible, cord. The receptacle on the knife was three-pole with one longer pin used for the ground wire. The male receptacle pins are enclosed and recessed in a yellow housing that is pressed into a double threaded, metal outer connector.

The metal portion of the receptacle is not connected to the ground wire. The receptacle, in this application, is screwed into a heat insulator that is also an excellent electrical insulator so the receptacle is totally isolated from any electrical ground.

The plant is a food processing plant and the equipment is cleaned with chlorine/cleanser and water on a regular basis. The moveable knife carriage is removed prior to cleaning. The flexible yellow cords are left in place.

Even though the cords with plug are wrapped in plastic, water gets into the plugs. The knife carriage is reinstalled after cleaning and the sometimes wet plugs are connected to the heater receptacles on the carriage.

The 277VAC power is turned on so the heaters can warm up to the running temperature and then is left on with an automatic control system maintaining the set temperature.. When there is water creating a path from one of the hot pins to the threaded metal housing of the receptacle, that receptacle housing becomes energized at somewhere between 1 and 277VAC to ground. If two different receptacles became energized at the same time, there could be a potential of 460VAC between them.

The mechanics have to work around the area and get both hands into the area where the receptacles are located. The technicians also have to work with both hands in the same area. This is a recipe for disaster. It looks safe, but can be extremely hazardous.

There had been several instances of shock accidents happening before I figured out what the cause of the electrical shocks were. The Eagle operators, shift mechanics and the shift technicians are the employees at intermittent risk from this hazard.

I was changing an RTD temperature detector/sender on the knife carriage. There are three RTD's monitoring three pairs of heaters installed on the knife carriage. In order to change the RTD, it has to be pushed and twisted to disengage the cap from the holder.

We have devised two different methods of grounding the metal receptacle part of the heater receptacle to eliminate the shock hazard. The first is rather awkward that involves putting a grounding ring under the metal part when installing on the insulator and then running a ground wire to the metal carriage. The second is to drill a hole in the metal casing down to the ground pin and then filling with solder.

Neither of these solutions is ideal. The ideal solution would be for the manufacturer, Woodhead, to ground the metal portion of the current carrying receptacle to the ground wire at the factory during manufacture of the part. I believe such a situation is addressed in both the NEC and NEMA Standards.

The potential risk for employees can be reduced through turning off the heaters before working in the area. This does not eliminate the hazard.

Any metal part of manufactured equipment that has the potential to become energized from a current-carrying voltage source must be grounded. If it can go to ground, Murphy's Law says that you can be assured that at some point, it will go to ground.

The part under discussion, the three pole receptacle, has a metal, threaded housing that would normally be screwed into a metal receiver of some sort, such as a metal box, that would be grounded, thereby grounding the threaded housing on the receptacle.

The grounding would be through the box to the building ground and the shock threat would be eliminated. Due to the high heat requirements of the knife heaters, the receptacle is screwed into a heat and electrical insulator.

The three pole rubber cord with female plug has a ground wire that mates with the longest pin in the receptacle. This is by definition and use the grounding pin of the receptacle and should be marked as ground and connected to the surrounding threaded metal part of the receptacle.

As a follow-up, I checked in the National Electric Code Handbook and found the following information concerning the grounds:

Article 250.4 General Requirements for Grounding and Bonding.

(A) Grounded Systems.

(2) Grounding of Electrical Equipment.
Non-current carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to earth during normal operation.

(4) Bonding of Electrically Conductive Materials and Other Equipment.
Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.

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Author: Larry Bush 
Profile: Electrician in industrial, construction, marine, and food industries for 47 years, with 22 of those years in management.
URL: http://www.reliability-consultant.com