Under a load is used in the electrical measurement technology and electronics , a load of a measuring instrument understood that provides an electrical output signal such as in transmitters , or transducers , namely a load in the sense of a load resistor . The term burden encompasses the entire DC and AC resistance of the connecting cables and devices that are externally connected to the output terminals.
In many cases, these measuring devices behave like a power source . Due to their special character, the greater the load resistance, the more unfavorable the situation: While with a voltage source the short circuit is the operating state to be avoided, with a current source the no-load operation due to open connection terminals is to be avoided, i.e. the infinitely large load resistance; In the jargon one speaks of an overload due to an excessive burden .
The term burden is mainly used in the field of electrical power engineering and automation technology (for devices with standard signals ). There is also the term burden or burden capacitance as an occasional designation of the load capacitance of quartz crystals operated in parallel resonance .
Load in power engineering
In the case of current transformers in electrical power engineering and high-voltage engineering , the burden is an unavoidable and necessary companion when measuring the (translated) current. The burden can appear as an ohmic resistance or as an impedance with an inductive component. For example, an ammeter can be used as a direct burden . Some versions of accessory clip-on ammeters for multimeters (i.e. those without a built-in display) are also designed like current transformers.
Current transformers must meet certain accuracy requirements, as they are used, among other things, to measure electrical energy and bill it. The burden must keep to a given range so that the measurement error remains sufficiently small. For current transformers, permissible load values are specified in the EN 61869 standard in order to maintain an accuracy class. The term rated burden (in earlier standards nominal burden) describes the maximum permissible load on the current transformer. It is specified as apparent power in the unit volt amperes (VA), formed as the product of the nominal current occurring on the secondary side of the current transformer (5 A is usual) and the associated voltage drop at the load. The values for commercial loads range between 2.5 VA and 30 VA.
Even with voltage transformers , the burden, the impedance of the secondary circuit, is usually expressed by the apparent power that is absorbed under specified conditions.
Burden in automation technology
Another area of application is automation technology and 4 to 20 mA technology in accordance with DIN IEC 60381-1 in the form of an analog two-wire circuit for sensor signals. The range of values for the current intensity of this circuit is between 4 and 20 mA. The current display device shown in the picture and the resistance of the transmission lines of different lengths in each installation represent the burden that should have a negligible influence on the current strength within a permissible range. In order to comply with the specifications of the standard signal (and thus to ensure the correct functioning of the sensors supplied via the line), the burden must not be too high. If the converter in the circuit shown requires a minimum supply voltage of 10 V, the load may be up to 700 Ω at a supply voltage of 24 V, which results in a voltage loss of up to (24−10) V at 20 mA.
- Rupert Patzelt, Hans W. Fürst: Electrical measurement technology . Springer, 1993, ISBN 3-211-82442-1 , chapter 2.6 - transducers.
- IEC 60050, see DKE German Commission for Electrical, Electronic and Information Technologies in DIN and VDE: International Electrotechnical Dictionary. Entry 321-01-25.
- DIN EN 60688: 2013-08 Electrical measuring transducers for converting electrical alternating current quantities and direct current quantities into analog or digital signals. No. 3.1.9.
- Kurt Bergmann: Electrical measurement technology. Vieweg, 1993, p. 71.
- Melchior Stöckl, Karl Heinz Winterling: Electrical measurement technology. Teubner, 1987, p. 168.
- Hans-Hellmuth Cuno: Practical Electronics. (PDF) VIII oscillator circuits. September 8, 2003, p. 7 , accessed on April 11, 2016 : "The capacitance connected in parallel to the quartz is called the burden."
- DIN EN 61869-1: 2010-04, also VDE 0414-9-1: Instrument transformers - Part 1: General requirements.
- DIN EN 61869-2: 2013-07, also VDE 0414-9-2: Instrument transformers - Part 2: Additional requirements for current transformers.
- DIN IEC 60381-1: 1985-11: Analog signals for regulation and control systems; Analog DC signals.
- From the company documents (PDF) page 7, image 12; accessed on June 15, 2016.
- From the works documentation (PDF) pages 3 and 7; accessed on June 15, 2016.