NFPA 79 Disconnecting Means

[cjosey]

I would like some interpretation on the following paragraphs from NFPA 79, Electrical Standard for Electrical Machinery.

Paragraph 7.8.1, Exception No. 2, states that machines with a motor(s) totaling two horsepower or less shall be permitted to be connected to a remotely mounted disconnect means through a flexible cord, cable, or conduit provided the disconnecting means is in sight from, readily accessible to, and no more than 20 ft from the machine operator. Paragraph 7.9.1, Exception No. 4 states that interlocking shall not be required provided that a tool is required to open the enclosure door and a label is attached to the control enclosure warning of dangerous voltage inside and advising disconnection of the power before opening.

Paragraph 11.8, states that any door(s) that permits access to live parts operating at 50 volts or more shall be so interlocked that the door(s) cannot be opened unless all power is disconnected. However, Exception No. 4 states that where the motor(s) on the machine totals two horsepower or less, an external, noninterlocked disconnecting means shall be permitted provided that the disconnecting means is in sight from and readily accessible, the control enclosure door or cover is marked with a warning indicating that power shall be removed by the disconnecting means before the enclosure is opened, and further provided that a tool is required to open the enclosure.

The only motors on our products are on pumps. Motors could (hopefully) be interpreted to mean only those that turn or move equipment, not pumps. Pumps are not inherently dangerous, so there should be no reason to put a limit on their sizing. (FYI, each pump has its own disconnecting means.) I would like to remotely mount our disconnecting means and not interlock our tool's electrical enclosures.

What do you think? Am I just reaching here?

Carl Josey

 

[dbaird]

Personally, I would ALWAYS interlock any electrical enclosure per SEMI S2-93 standards 5.4 which prevents access to hazardous voltage above: "30 RMS, 42.4Vpeak, 60Vdc". If a physical barrier is not provided that requires a tool to obtain access, an interlock is mandatory. As engineers, designers and managers I feel it is our responsibility to try and prevent injury or damage due to Electric shock. One question to always ask is it foreseeable that the hazard could cause injury by entry of untrained and/or irresponsible persons whether troubleshooting or just being quizzical. Any enclosure door with lethal voltages that can be yanked open at any time without some sort of prevention to me is just begging for an accident and law suits. Especially around pump/piping-wet environment. Remember:

Readings
Safe Current Values

1 mA or less
Causes no sensation - not felt.

1 mA to 8 mA
Sensation of shock, not painful;Individual can let go at will since muscular control is not lost.

Unsafe current values

8 mA to 15 mA
Painful shock; individual can let go at will since
muscular control is not lost.

15 mA to 20 mA
Painful shock; control of adjacent muscles lost; victim can not let go.

50 mA to 100 mA
Ventricular fibrillation - a heart
condition that can result in death - is possible.

100 mA to 200 mA
Ventricular fibrillation occurs.

200 mA and over
Servere burns, severe muscular contractions – so severe that chest muscles clamp the heart and stop it for the duration of the shock. (This prevents ventricular fibrillation).
David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.