While most people are aware of the danger from electric shock, few realise how little current and how low a voltage are required for a fatal shock. Current flows as low as 30 mA can be fatal.
The effects of current flow through a 68 kg male include:
* At about 10 mA, muscular paralysis of the arms occurs - he cannot release his grip.
* At about 30 mA, respiratory paralysis occurs - breathing stops and the results are often fatal.
* At about 75 to 250 mA, for exposure exceeding five seconds, ventricular fibrillation occurs, causing discoordination of the heart muscles; the heart can no longer function. Higher currents cause fibrillation at less than five seconds. The results are often fatal.
Now let us calculate the threshold for a 'hazardous' voltage. The approximate body resistance under the skin from hand to hand across the body is 1000 Ω. A voltage of only 30 V across 1000 Ω will cause a current flow of 30 mA. Fortunately, the skin's resistance is much higher. It is the resistance of the skin, especially the outer layer of dead cells, called the 'horny layer,' that protects the body. Under wet conditions, or if there is a cut, skin resistance drops radically. At about 600 V, the resistance of the skin ceases to exist. It is punctured by the high voltage.
For multimeter manufacturers and users, the objective is to prevent accidental contact with live circuits at all costs. Look for:
* Meters and test leads with double insulation.
* Meters with recessed input jacks and test leads with shrouded input connectors.
* Test leads with finger guards and a non-slip surface.
* Meter and test leads made of high-quality, durable, non-conductive materials.
Choosing a multimeter is like anything else - you get what you pay for. You may think you are safe if you choose a multimeter with a high-enough voltage rating for a particular application. Engineers who analyse multimeter safety often discover that failed units were subjected to a much higher voltage than the user thought he was measuring. There are occasional accidents when a meter rated for low voltage (1000 V or less) was used to measure medium voltage, such as 4160 V. Just as common is the knock-out blow that had nothing to do with misuse - it was a momentary high-voltage spike or transient that hit the multimeter input without warning.
Voltage spikes and transients
As distribution systems and loads become more complex, the possibility of transient overvoltages increases. Motors, capacitors and power conversion equipment can be prime generators of spikes, as can lightning on outdoor lines. If you are taking measurements, these transients are invisible and largely unavoidable hazards. They occur regularly on low voltage power circuits and can reach many thousands of volts. Here, your protection is dependent on the safety margin built into your meter. The voltage rating alone will not tell you how well that meter was designed to survive high transient impulses.
The real issue for multimeter circuit protection is a combination of both steady state and transient overvoltage withstand capability. Transient protection is vital. When transients ride on high-energy circuits, they tend to be more dangerous because these circuits can deliver large currents. If a transient causes an arc-over, the high current can sustain the arc, producing a plasma breakdown or explosion, which occurs when the surrounding air becomes ionised and conductive. The result is an arc blast, a disastrous event that causes more electrical injuries every year than the better known hazard of electric shock.
Overvoltage installation categories
The most important single concept to understand about standards is the Overvoltage Installation Category. This standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc. The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (AC resistance) of the system. A higher CAT number refers to an electrical environment with higher power available and higher-energy transients. Thus a multimeter designed to a CAT III standard is resistant to much higher-energy transients than one designed to CAT II standards.
Within a category, a higher voltage rating denotes a higher transient withstand rating, eg, a CAT III-1000 V meter has superior protection to a CAT III-600 V rated meter. The real misunderstanding occurs if someone selects a CAT II-1000 V rated meter thinking it superior to a CAT III-600 V meter.
Not just the voltage level
An overhead line run from a house to a detached workshed might be only 120 V or 240 V, but it is still technically CAT IV. Why? Any outdoor conductor is subject to very high-energy lightning-related transients. Even conductors buried underground are CAT IV, because although they will not be directly struck by lightning, a lightning strike nearby can induce a transient because of the presence of high electro-magnetic fields.
Transients - the hidden danger
A technician performing measurements on a live three-phase motor control circuit, using a meter without the necessary safety precautions, could experience the following:
1. A lightning strike causes a transient on the power line, which in turn strikes an arc between the input terminals inside the meter. The circuits and components to prevent this event have failed or are missing. Perhaps it was not a CAT III rated meter. The result is a direct short between the two measurement terminals through the meter and the test leads.
2. A high-fault current - possibly several thousands of amps - flows in the short circuit just created. This happens in thousandths of a second. When the arc forms inside the meter, a very high-pressure shock wave can cause a loud bang. At the same instant, the tech sees bright blue arc flashes at the test lead tips - the fault currents superheat the probe tips, which start to burn away, drawing an arc from the contact point to the probe.
3. The natural reaction is to pull back, in order to break contact with the hot circuit. As the hands are pulled back, an arc is drawn from the motor terminal to each probe. If these join to form a single arc, there is now another direct phase-to-phase short, this time directly between the motor terminals.
4. This arc can have a temperature approaching 6000°C - higher than an oxy-acetylene cutting torch. As the arc grows, fed by available short circuit current, it superheats the surrounding air. Both a shock blast and a plasma fireball are created. If the technician is lucky, the shock blast blows him away and removes him from the proximity of the arc; though injured, his life is saved. In the worst case, the victim is subjected to fatal burn injuries from the fierce heat of the arc or plasma blast.
In addition to using an appropriately rated multimeter, users should be protected with flame resistant clothing, should wear safety glasses or, better yet, a safety face shield, and should use insulated gloves.
Transients are not the only source of possible short circuits and arc blast hazard. One of the most common misuses of handheld multimeters can cause a similar chain of events.
Let us say a user is making current measurements on signal circuits. The procedure is to select the amps function, insert the leads in the mA or amps input terminals, open the circuit and take a series measurement. In a series circuit, current is always the same. The input impedance of the amps circuit must be low enough so that it does not affect the series circuit's current. The input impedance on the 10 A terminal of a Fluke meter is 0,01 Ω. Compare this with the input impedance on the voltage terminals of 10 MΩ. If the test leads are left in the amps terminals and then accidentally connected across a voltage source, the low input impedance becomes a short circuit! It does not matter if the selector dial is turned to volts; the leads are still physically connected to a low-impedance circuit. (Some multimeters, such as the Fluke 180 Series, have an input alert that gives a warning beep if the meter is in this configuration.) That is why the amps terminals must be protected by fuses. Those fuses are the only thing standing between an inconveniently-blown fuse and a potential disaster. Use only a multimeter with amps inputs protected by high energy fuses. Never replace a blown fuse with the wrong fuse. Use only the high-energy fuses specified by the manufacturer. These fuses are rated at a voltage and with a short circuit interrupting capacity designed for your safety.
Overload protection
Fuses protect against overcurrent. The high input impedance of the V/Ω terminals ensures that an overcurrent condition is unlikely, so fuses are not necessary. Overvoltage protection, on the other hand, is required. It is provided by a protection circuit that clamps high voltages to an acceptable level. In addition, a thermal protection circuit detects an overvoltage condition, protects the meter until the condition is removed, and then automatically returns to normal operation. The most common benefit is to protect the multimeter from overloads when it is in ohms mode. In this way, overload protection with automatic recovery is provided for all measurement functions as long as the leads are in the voltage-input terminals.
Shortcuts to understanding categories
Here are some quick ways to apply the concept of categories to everyday work:
* The closer you are to the power source, the higher the category number, and the greater the potential danger from transients.
* The greater the short-circuit current available at a particular point, the higher the CAT number.
* The greater the source impedance, the lower the CAT number. Source impedance is simply the total impedance, including the impedance of the wiring, between where you are measuring and the power source. This impedance is what dampens transients.
* Finally, if you have any experience with the application of TVSS (transient voltage surge suppression) devices, you understand that a TVSS device installed at a panel must have higher energy-handling capacity than one installed right at the computer. In CAT terminology, the panelboard TVSS is a CAT III application, and the computer is a receptacle connected load and, therefore, a CAT II installation. As you can see, the concept of categories is not new and exotic. It is simply an extension of the same common-sense concepts that people who work with electricity professionally apply every day.
On the whole, it is not realistic to expect people to be going through the category-defining process all the time. What is realistic, and highly recommended, is to select a multimeter rated to the highest category in which it could possibly be used. In other words, err on the side of safety.
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