"Ground Connected" lamp a safety hazard?
"Ground Connected" lamp a safety hazard?
(OP)
We manufacture industrial automation that is asscociated with mechanical power presses and we have a customer who has a specification that reads "A suitable detector lamp shall be provided to verify ground continuity. The pilot light shall be a transformer push-to-test." Is this a code violation? Could this create a shock hazard? I have heard that it is a code violation in accordance the Canadian Electric Code, but not sure if it violates NEC.





RE: "Ground Connected" lamp a safety hazard?
RE: "Ground Connected" lamp a safety hazard?
RE: "Ground Connected" lamp a safety hazard?
RE: "Ground Connected" lamp a safety hazard?
Sorry I can not give you an NEC reference on this but if I get time I will look it up.
RE: "Ground Connected" lamp a safety hazard?
The push to test is just simply a momentary bypass from the cold end of the transformer primary to the neutral terminal of the transformer in order to distinguish between an open bonding wire and a burned out bulb.
In general, National Electrical Code prohibits using the equipment ground as a current carrying conductor. One exception is for 240 volt corner grounded bridge cranes that are supplied from a dedicated transformer and use one of the runway rails as the grounded circuit conductor like streetcars do. The other is for existing electric stove and dryer circuits.
There is no explicit application or rule covering this other than National Electrical Code 90.4 that permits alternate methods of acheiving the same safety goal and specialized applications that are not explicitly covered.
Mike Cole, mc5w@earthlink.net
RE: "Ground Connected" lamp a safety hazard?
lamp
L1-----------( )-NC-----GROUND
\
NO----------------N
<-------------120VAC------------->
RE: "Ground Connected" lamp a safety hazard?
In the spirit of NEC you can not use ground conductor as 'curret carrying' conductor intentionally. However if used only momentarily as a test function with adequate imepdance ( a resistor) in series with the lamp so that only few mA flows, just enough to make the lamp glow..it should be acceptable, imho. The resittor will protect against a shorted lamp..which could be very dangerous if there is no other impedance to limit the current.
I beleive you can easily find a push to test lamp assembly using LED and a resistor.
RE: "Ground Connected" lamp a safety hazard?
We use something like this on all of our portable and mobile equipment in the mining industry. The device is called a "Ground wire monitor". It is a low voltage circuit that sends a signal up a wire called a "pilot wire". The signal is returned via the ground wire. If the circuit is complete, you can light a lamp or operate a relay etc. If either the pilot wire or the ground wire is opened, the ground wire monitor senses the open and de-energizes its' output. It is a fail-safe design. The "push-to-test" feature is done in the pilot wire, and not in the ground wire.
Certain of these ground wire monitors are approved to deal with parallel grounding paths, and may be more sophisicated that what you may need, but hey are made by companies such as Service Machine Comapny, Line Power Inc. etc. I can provide contact information for these companies if you are interested in pursuing this option.
Regards,
Dave
RE: "Ground Connected" lamp a safety hazard?
an alternaltive to the ground checking circuit might be use of a ground fault protection.
good luck,
RE: "Ground Connected" lamp a safety hazard?
RE: "Ground Connected" lamp a safety hazard?
loss of grounding is a common industrial problem.
in high demand locations (100's of circuits) automatic ground fault locators are available.
they work in both floating neutral and grounded neutral systems.
good luck
RE: "Ground Connected" lamp a safety hazard?
I am in a mining enviroment, and we use a so called "earth continuity monitor" on our electrical equipment. The equipment is mobile, and the only earthpoint is through the trailing cable. The monitor send a signal through the pilot wire in the cable, via a 100ohm resistor and back through the earth wire. You can gave the monitor a low setting (100 ohm) and a high setting (120 ohm, 20 ohm for cable resistance) and if the reading is between the setpoints, the earth is still fine. For a open circuit the resistance is infinity. For a short circuit between the pilot and earth in the cable, the resistance is very low.
The monitor sensor the loop-resistance continuously, and trip for any open circuit in the loop, or for a short circuit (between pilot and earth) in the cable. The short-circuit function is to prevent anyone to "bridge" the pilot and earth wires
On this moment I do not have the contact-details of the manufacturer, but I will post it if I am back at work.
Regards
Ralph
RE: "Ground Connected" lamp a safety hazard?
www.switchingsystems.co.za
www.switchingsystems.co.za/products/ecp_earth_cont...
On our old systems we've used a normal signal through pilot and earth to detect if the earth-wire was still connected to the equipment. Then, a few years ago, there was a fatal accident. (Someone was electrocuted) What actually happened was that the trailing cable was damaged, in such a way that the red face melted to the earth wire. But somewhere in the cable, between the source and the damaged place, the earthwire was broken. Now someone must have seen that the loop was open, but was to lazy to search for the devective (broken) place, and just bridged the pilot and earth at the source. Now the machine was not earhed, and was also alive. The now deseased went to the machine, and when he touched the frame he was the link to earth, and all the current went through him. He was killed instantly. To prevent this kind of thing to happen again, we've decided to use the monitor with the resistor.
As a last note, the voltage was 6.6kV, and the trailing cables is up to 1km long. But you can still use the device at lower voltages, the concept stays the same.
Regards
Ralph