Difference between revisions of "Neon screwdriver"
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Mains power flows down the screw driving tip to the resistor and neon in the handle. When you touch the brass cap, the bodies natural capacitance allows enough power to flow through the screwdriver to cause the neon to light. | Mains power flows down the screw driving tip to the resistor and neon in the handle. When you touch the brass cap, the bodies natural capacitance allows enough power to flow through the screwdriver to cause the neon to light. | ||
| − | The currents involved are very small, so you will not | + | The currents involved are very small, so you will not feel any sense of electric shock under normal circumstances. |
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==The Problems== | ==The Problems== | ||
Revision as of 20:10, 15 March 2012
Neon screwdrivers are fairly crude and simple instruments that combine the functions of a small terminal screwdriver with a mains voltage detector. Note however, they have a number of potential safety problems, and the majority view on uk.d-i-y is that the voltage detection capability is best not used at all.
How they work
Mains power flows down the screw driving tip to the resistor and neon in the handle. When you touch the brass cap, the bodies natural capacitance allows enough power to flow through the screwdriver to cause the neon to light.
The currents involved are very small, so you will not feel any sense of electric shock under normal circumstances.
The Problems
Direct shock
Water or condensation on the resistor can result in the tool passing higher current to the user than intended, causing a shock, maybe a fall from a ladder. Hence never hold one of these screwdrivers in your mouth when working with both hands for example.
Certain tool faults such as resistor a failure can create similar risks.
False negative
The lighting of the neon is sometimes too dim to see, especially in daylight. The result is shock from touching a live wire, and this is probably the most common failure mechanism.
Not being connected to or near earth, insulated from ground, can reduce current flow further, reducing light output and making the dimness situation worse. The body being capacitively coupled to some other nearby live conductor, can prevent the light lighting at all, even when the screw driver tips is touching a live wire or contact.
False Positive
A user capacitively connected to a live supply will light a neon screwdriver touched to any non-live wire. It is quite easy to be capacitively coupled in this way without noticing. Holding an inspection light, standing on a cable, even leaning on a wall in some cases, and so on. Also touching a "floating" conductor (i.e. an uncontaminated wire that is routed in close proximity to other live wires) may also yield a false positive test.
Minimising risk / safe use
If you must use one of these devices, then adobe a sound "prove dead" approach to testing. Before making a test, test the driver on a known live conductor and ensure it lights up. Next test the unknown conductor, and ensure it does not light, Finally test again on a known life conductor.
Legality
There is some question as to whether these tools, are legal to use in the workplace, since they don't possess a number of the attributes deemed necessary in electrical test gear by the health and safety executive. See the document GS38 Electrical Test Equipment for Electricians (PDF) - in particular section 9.
Further discussion
Alternatives
A much better alternative is a non contact volt stick:
These can give a clear and unambiguous indication of a live circuit, without even needing to open any insulating cover or make physical contact with the circuit.
Other alternatives a multimeter or other specialist electricians voltage meter. A reasonable quality meter will do far more than a neon screwdriver, and do so with more reliable results. Multimeters have their own safety issues, but they are less and much more under the control of the end user, so can be minimised with care.