"Ground Connected" lamp a safety hazard?

[EEAutomEng]

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.

 

[EEAutomEng]

Forgot to add something...nothing I could find in NFPA 79 that says anything about this.

 

[buzzp]

I don't think it is a violation since it is not continuously energized (speaking of US codes). I could be wrong. These types of circuits are used extensively in the mining industry. There are several handheld units that will check continuity and I believe they work on using reflected waves (very small current) whereas what your talking about sounds like it would be energized using a higher voltage/current. This could be asking for trouble. I am sure someone else will be able to help more on this subject.

 

[EEAutomEng]

It is connected to the secondary of a 1.5 KVA transformer. Voltage is 120 VAC 60 HZ. The neutral (X2) is grounded.

 

[buzzp]

Yeah that sounds a little dangerous to me. Whatever current the light takes is what will be passed through the wire @120VAC. Just hope no one can access this wire when someone decides to test it!
Sorry I can not give you an NEC reference on this but if I get time I will look it up.

 

[mc5w]

I have experience with this lamp circuit in punch press applications. This is just a pilot lamp that runs from the hot wire to the frame of the machine and it indicates whether or not the bond wire from neutral to the machine froam is intact. For punch presses this is a serious issue because vibration can break or loosen connections even if the control box is floor mounted. For a transformer type pilot light there are no safety issues when relamping.

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

 

[EEAutomEng]

I have wired this circuit before when I first learned panel building, but never understood why it was done. Typically, a control transformer would be mounted on top of the enclosure and the push-to-test pilot lamp would be mounted on one of the doors of the enclosure. The neutral from the transformer and the normally open contact of the lamp would be connected to the same ground bar on the sub-panel. The normally closed contact of the lamp would be connected to the ground bar. The push-to-test function would serve to connect the lamp to neutral instead of the ground wire so you could verify that the bulb is working.

lamp
L1-----------( )-NC-----GROUND
\
NO----------------N

<-------------120VAC------------->

 

[rbulsara]

Speaking of NEC codes, this may be a grey area..

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.

 

[raisinbran]

JMBurgess,
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

 

[hacksaw]

the NEC specifically deals with fire protection. having a ground test is not necessarily a violation of that code if it is a test circuit and incapable of starting a fire or causing electrical shock. additionally, as a code it only defines the minimum requirements in cases where it is subscribed to by local jurisdictions.

an alternaltive to the ground checking circuit might be use of a ground fault protection.

good luck,

 

[mc5w]

Ground fault protection will not work if the power supply somehow becomes ungrounded due to a loose or broken bonding jumper.

 

[hacksaw]

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

 

[RalphChristie]

JMBurgess:

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

 

[RalphChristie]

Earth continuity monitor:

http://www.switchingsystems.co.za

http://www.switchingsystems.co.za/products/ecp_earth_continuity_monitors.htm

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