Electrical Hazards

Safety is an important topic in ham radio.  There are 11 questions on electrical hazards in the USA Technician class license exam pool, 13 questions on tower safety and associated grounding, and 13 questions on radio frequency (RF) hazards.

Several of these have been used by us previously but in retrospect we should have given the safety topic more airtime, pun intended.  New hams are unlikely to have antenna towers so we don’t plan to discuss tower safety much.  That leaves electrical and RF hazards to cover.

This post will address general electrical hazards and related safety; a future post will focus on RF hazards.

Radios and accessories are electrical devices so let’s start with the most obvious hazard: electric shock, which is caused by current flowing through a human body.  Current is useful in electronics but harmful when flowing through a person.  Current can disrupt heart and lung function at even low levels.  It can also cause unwanted muscle movement, or prevent it (can’t let go).  At higher levels, electric current will damage skin and internal organs.

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There are many factors in electric shock and there are other electrical hazards.  But this is a big one and you should avoid touching live circuits.

Fire is another electrical hazard.  When too much current flows in conductors, the wires can get very hot and ignite combustible material.  In fact, the US National Electrical Code is actually a document of the National Fire Protection Association (NFPA), not a government agency.

To limit the risk of fire and other damage, every power circuit needs some form of  protection.  Fuses are quite common; their internal metal melts at a pre-determined current to disconnect power.

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Also, a smaller (amp rating) fuse can safely be inserted in a protective circuit but one should never put in a larger one.  A fuse is sized to the circuit requirements and wiring  is sized to the fuse.  So a higher-ampacity fuse will not properly protect the wires or the circuit and serious overheating may occur in both AC and DC power circuits.

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In addition to one-time use fuses, circuit breakers are another popular form of circuit protection; these may be reset and are often used as an on/off switch.

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While fuses and circuit breakers do not directly provide shock protection, they may do so Continue reading

Flat Ground Strap

Ever wonder why RF grounds should be flat straps and not regular wires?

This is because ordinary wires are not good conductors at frequencies higher than 50-60Hz. This complicates wiring and bonding requirements.

Impedance (effectively, AC resistance) of a conductor increases with frequency and length due to inductive reactance.  The higher the frequency, the greater the impedance.

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All conductors have some measurable inductance, and it doesn’t take much to yield significant impedance.  At KHz or MHz frequencies, long round wires might present hundreds or even thousands of Ohms impedance; not suitable for grounding.

A good ground has less than one ohm impedance.  This is a genuine safety issue.

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Since inductive reactance increases with frequency and length, safety grounds and module bonds need to be something other than long round wires when radio frequencies are involved.

When high frequency grounding is required, use short, wide, and flat conductive straps.  The high aspect ratio minimizes electrical inductance vs. a round wire, as does a short conductor.  This lowers the ground wire’s impedance at higher frequencies.

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So now you know.  Keep it flat and short (KIFS is a lousy acronym).

It’s not just a suggestion; this one might just bite you if you don’t heed the guideline!