Multímetros

Los multímetros son la herramienta más importante cuando trabajando con electricidad. Un multímetro le permite medir todas las características importantes de un circuito o sistema eléctrico, lo que le permite trabajar de forma segura y verificar si un circuito funciona correctamente. Hay muchos tipos diferentes de multímetros que ofrecen una variedad de funciones diferentes. Asimismo, varían en costo dramáticamente. Todos los multímetros, incluso los más baratos, deberían poder realizar las medidas básicas necesarias para realizar trabajos eléctricos: voltaje, corriente, resistencia, conductividad y frecuencia. Dicho esto, es importante asegurarse de que el multímetro que está utilizando esté calificado para usarse con el voltaje y la corriente potenciales más altos que desea medir. Si un multímetro no está clasificado para funcionar en las condiciones de uso, podría destruir el multímetro y causar daños graves. Los multímetros se clasifican en varias categorías que informan a los usuarios sobre su capacidad para manejar el exceso de voltaje y corriente. Estas clasificaciones van desde la clasificación más baja que es la Categoría I (para uso con dispositivos electrónicos pequeños con fuentes de corriente y voltaje limitadas) a la Categoría IV (para uso en la conexión a la red pública y todos los conductores exteriores). Para pequeños sistemas fotovoltaicos fuera de la red Se recomienda al menos un multímetro Cat II - 600 V y debería ser bastante económico.

Funciones de un multímetro

Voltaje

Cualquier multímetro que se utilice tiene la capacidad de medir tanto CC como CA. Es importante conocer el tipo de corriente que se va a medir, ya que el uso de una configuración incorrecta puede generar una lectura falsa que puede ser engañosa y peligrosa. Un multímetro tendrá un dial que le permitirá al usuario seleccionar entre CA y CC, que se identificarán mediante los símbolos comunes que se ven a continuación. Cuando se trabaja con un sistema fotovoltaico, el voltaje generalmente se mide por las siguientes razones:

• Para comprobar si hay voltaje en un circuito antes de comenzar a trabajar.
• Para probar el rendimiento de un componente en un sistema FV. ¿Está el voltaje dentro de un rango aceptable dadas las condiciones? La batería debe estar completamente cargada, ¿cuál es su voltaje?

• Para identificar dónde se encuentra un problema en un sistema mediante la resolución de problemas probando varios circuitos y componentes del sistema, como la batería, la fuente FV, el inversor, los enchufes o las tomas de luz.

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  Voltaje CC

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  Voltaje CA

Corriente

No todos los multímetros pueden tomar medidas de corriente CA y CC, pero para trabajar con sistemas FV es importante encontrar uno que lo haga. Hay dos formas diferentes de medir la corriente con un multímetro, esto se explora en la sección - tipos de multímetros. Cuando se trabaja con un sistema FV, las mediciones de corriente se toman normalmente por las siguientes razones:

• Para verificar si hay corriente fluyendo en un circuito antes de comenzar a trabajar.
• Entender cómo está funcionando un módulo o una fuente FV. ¿Cuánta corriente está produciendo en las condiciones?
• Medir el consumo de energía de un sistema o varias cargas dentro de un sistema.
• Entender el flujo de electricidad. Si el sistema está alimentando una carga pesada, qué circuito o aparato está consumiendo tanta energía?

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  Corriente CC

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  Corriente CA

Resistencia

Todos los componentes de un sistema eléctrico tienen una cierta cantidad de resistencia interna, pero también resistencia relativa a otros componentes del sistema. Cuando se trabaja con un sistema fotovoltaico, las mediciones de resistencia generalmente se toman por las siguientes razones:

• Para garantizar que una conexión realizada en el sistema tenga una resistencia lo suficientemente baja para funcionar correctamente. Las conexiones de alta resistencia pueden generar calor e incendios.
• Asegurar que exista una alta resistencia entre el sistema de puesta a tierra y los componentes del sistema que no deben tener conexión de ningún tipo a tierra como un conductor sin conexion a tierra Una baja resistencia en este caso significa que hay un falla a tierra.

Seguridad: No se pueden tomar medidas de resistencia en circuitos que tienen voltaje o que tienen corriente fluyendo por ellos. No lo intente es peligroso.

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  Resistance

Conductivity

A conductivity check is a simplified resistance measurement. If a connection has a sufficiently low resistance - below a certain value that is set inside the multimeter - then the multimeter will emit a sound. If there is a high resistance between the two points of measurement, the multimeter will do nothing. Some common checks that are done with a conductivity test:

• Check whether a breaker, fuse or switch is working. A fuse should have conductivity if it is still good. If it fails a conductivity test then it needs to be replaced and the reason for its failure investigated.
• To test quickly whether a connection to a busbar or between two wires in a circuit was done properly.
• To test whether a wire or system component is a part of a circuit. Often times wires are not properly identified, but a quick conductivity can often identify where that wire leads and to what it is connected.

certain light or outlet.

Safety: Conductivity checks cannot be taken on circuits that have voltage or that have current flowing on them. Do not attempt it.

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  Continuity

Frequency

Frequency is a property of AC current. Inverters in an off-grid system should be operating within a specific frequency range, a frequency check can tell us that they are outside of this range and that there is a problem. AC frequency being out of this acceptable range can cause issues with appliances.

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  Frequency

Types of multimeters

Multimeter types:
Left - Clamp meter.
Center - Traditional autoranging handheld multimeter.
Right - Probes

Traditional handheld multimeter

These are the most common and economical multimeters on the market. For a low price they can perform many different functions and, if it is a decent brand, should be fairly accurate. These multimeters rely on probes to take all of measurements. These types of multimeters have at least three different ports and the probes must be rearranged depending upon the measurement. Taking voltage and current measurements - as seen in the diagram - always require the moving of the red probe to a different port as these two measurements use different circuits. As the multimeter measures current using probes, any current that is measured must pass through it, which is often limited to 10-15 A. There is an internal fuse that will be destroyed if one attempts to pass current in excess of its rated value through the multimeter. This means that one cannot take current measurements of circuits for which the maximum amount of current is not known. Nor can one take measurements of a source that can supply high amounts of current - like an outlet or a battery - as the multimeter simply provides a low resistance path for the current to pass through. If measuring currents near or in excess of 10A may be required, then it is necessary to get a clamp meter.

These meters come in two types:

Manual-ranging

This type of multimeter requires the user to select the maximum voltage, current, or resistance in order to take a measurement. If too high of a range is used, it will lead to inaccurate measurements. If too low of a range is used, the multimeter will give an error. The cheapest multimeters are manual-ranging. For example:

• Example 1: You would like to take a voltage measurement of a circuit that you suspect to be 220 V AC. Your options on the multimeter are 120 V AC, 250 V AC, 500 V AC. What is the proper range for taking this measurement?

Answer: 250 V AC. The 120 V AC setting will produce an error. The 500 V AC setting may produce an inaccurate reading. The 250 V AC setting will be able to provide the most accurate measurement.

One technique that works with a manual ranging multimeter, is to start at the highest range, get a measurement and then work your way down to the lowest range capable of measuring a value in that range until there is an error.

Auto-ranging

This type of multimeter does the work of selecting the range itself and can save time for the user. For a slightly higher price, one can purchase an auto-ranging multimeter. Auto-ranging multimeters cost slightly more than their manual-ranging counterparts, but the additional cost is worth it if you will be using the meter frequently.

Clamp meters

Clamp meters are auto-ranging and share all of the basic functions of a traditional handheld multimeter, but they enable current measurements above 10 A to a certain limit, typically around 300-400 A. The multimeter performs current measurements by measuring the current that is flowing in a wire based upon the electro-magnetic field that it generates. To take a measurement one simply passes one wire of a circuit through the jaws of the clamp, which can easily be opened and closed by a button the side. It is important to only pass one wire of a circuit through the clamp because if both wires are passed through the currents will cancel out.

A clamp meter is extremely useful, easy to use and significantly safer than a traditional handheld multimeter. The meters do cost slightly more, but the additional cost is is worth it if you will frequently take current measurements.

Safely taking measurements with a multimeter

Like any tool, a multimeter is at best useless if the person using it doesn't know how to do so properly and at its worst can be dangerous. There are a few basic questions that should be asked and steps that should be taken when performing any measurements with a multimeter.

1. What type of current are you working with?
2. What is maximum amount of voltage and current that the circuit you want to measure could supply?
3. Do you have the proper training and tools to work on this equipment?
4. What do you want to measure and why?
5. What value should you get if everything is as it should be?
6. Check the voltage of the circuit before performing any other measurements to check if it is live.
7. Set your multimeter to the right setting.
8. Carefully take the measurement and then check it against the anticipated value that you came up with in Step 5.