As an industrial instrumentation and electronics technician, i am sharing to you these safety notes and practices shared by Fluke (maker of my issued digital multimeter). Here it goes...
Three levels of electrical reaction:
You should be aware of the electrical shock levels at which you are likely to have a mild reaction. Higher levels may be accident-provoking or even fatal. (Current rather than voltage is used below because the reaction to voltage varies more from person to person (body impedance changes with contact condition - wet or dry, extent of bodily contact, etc).)
Perception Level. This refers to the ability of the operator to sense an electrical shock. Levels as low as 1 mA current with a 1-second contact can be detected as a slight warm in the palm of the hand (dc) and a mild tingling sensation (ac).
Surprise Level. The reaction caused by the surprise level current is sudden, involuntary and totally without warning. It will trigger an unexpected reaction such as suddenly pulling one's hand away from the source of shock with enough force to cause an accident. The reaction becomes more violent with higher levels of current and voltage. Specific surprise levels vary from person to person.
Freeze Level. As voltage and amperage levels increase, the "freeze level" is reached. The hand muscles contract to the point where the victim has no control. Inability to let go of the conductor could "freeze" the victim to the electrical source long enough to cause lung and heart stoppage.
Electrical circuits can cause burns, heart fibrillation and eye injuries.
The DMM user working with or near high voltage, high-current electricity is exposed to potential danger from both actual contact and from exposure to flash hazards. Electrical contact may cause burns either two ways: (1) by touching a high voltage supply, when the resulting arc causes sudden tremendous heat and a burn; or (2) by touching a high frequency current, causing a burn under the skin.
The heart can go into fibrillation when sufficient current passes through it; it seldom recovers spontaneously.
Flash hazards occur when a short circuit creates an arc. Shorts in high-energy circuits can cause the equivalent of welding arcs, which can flash-burn the eyes or cause injury from flying molten metal.
Electrical shock danger varies by wet or dry contact.
Dry skin has a resistance perhaps 10 times greater than wet skin. (Typical impedance of a wet gripping contact is 1500 ohms in parallel with 0.22 uF. Internal body resistance between hands is about 400 ohms.) Perspiration greatly decreases the meter's users skin resistance. The same voltage that would be safe with dry skin may put the user at the "surprise level" with wet skin.
Industrial high energy circuits (>220V ac).
The dividing line between high and low energy circuits is often given as 3,600 volt-amps (such as 240V ac, 15A circuit). Above this, a poorly designed DMM (digital multimeter), or one designed for light duty, could break down, resulting in damage and possible operator injuries. Only meters having special built-in protection such 600V fusing, ohms overload capability and transient protection should be used in these conditions.
Fault Conditions. These can occur when a circuit is accidentally shorted, or the DMM itself is overloaded. Faults cause arcing, sparks, fire explosions or other immediate hazards. (A simple overload is not a fault.) Common situations that lead to faults are:
- Accidental contact with a 480V ac power line while in the current mode.
- Accidental contact with power lines while in the resistance mode.
- High voltage transients from motor switching, load change, or indirect lightning surges.
Hazardous locations (potentially explosive atmospheres). A DMM may be safe by itself under normal operating conditions. But if it were to fault, it potentially could cause a spark or overheat a component to cause an explosion. Only specially designed and approved (and clearly labeled as such) meters should be used in hazardous locations. Even simply disconnecting test leads from live circuits on explosive environments, especially while measuring current with inductive loads, may create a dangerous spark.
Proper procedures for using DMMs
Always:
1) Inspect the test leads for insulation damage or exposed metal. Damaged leads should be replaced.
2) Reduce the risk of accidental contact by using leads with shrouded connectors and finger guards, and meters with recessed jacks.
3) Check continuity of the test leads.
4) Be certain the DMM itself is in good condition. During the continuity test, a meter reading that goes from overload to 0 generally means the circuitry is working properly.
5) Select the proper function and range for your measurement.
6) Electrically disconnect "hot" end first.
Safe Practices:
1) Insulate yourself from earth ground, using an insulating floor mat. If possible, employ one hand operation, keeping the other hand in your pocket to avoid accidentally allowing current to travel through your body to earth ground.
2) Follow all equipment safety procedures, disconnecting the power and discharging high-voltage capacitors prior to testing with the DMM. (This applies to dc power supplies, tube circuit transmitters, motor control circuits and x-ray machines.)
3) Working with a CRT? wear safety glasses and protective clothing to avoid injuries should an implosion cause flying glass.
4) Avoid working alone.
5) Using a current shunt? Turn the power off before connecting into the circuit. Overloading a current shunt will cause excessive heating.
6) Current greater than 2 Amps? Use clamp-on probes for maximum protection.
7) Measuring current transformer output or motor winding current? Check the fuse first. An open fuse will allow high voltage buildup, potentially damaging to equipment and operator.
8) Measuring automotive circuits? Be aware of potential danger from high voltage (up to 30,000V dc) and fire hazard from gasoline fumes/leakage.
