circuit. Examples of devices that can provide this isolation are Hall effect current sensors and current transformers (see clamp meters). Current shunts are considered more accurate and cheaper than Hall effect devices. Common accuracy specifications of such devices are ±0.1%, ±0.25% or ±0.5%. The Thomas-type double manganin walled shunt and MI type (improved Thomas-type design) were used by NIST and other standards laboratories as the legal reference of an ohm until superseded in 1990 by the quantum Hall effect. Thomas-type shunts are still used as secondary standards to take very accurate current measurements, as using quantum Hall effect is a time-consuming process. The accuracy of these types of shunts is measured in the ppm and sub-ppm scale of drift per year of set resistance. Where the circuit is grounded (earthed) on one side, a current measuring shunt can be inserted either in the ungrounded conductor or in the grounded conductor. A shunt in the ungrounded conductor must be insulated for the full circuit voltage to ground; the measuring instrument must be inherently isolated from ground or must include a resistive voltage divider or an isolation amplifier between the relatively high common-mode voltage and lower voltages inside the instrument. A shunt in the grounded conductor may not detect leakage current that bypasses the shunt, but it will not experience high common-mode voltage to ground. The load is removed from a direct path to ground, which may create problems for control circuitry, result in unwanted emissions, or both. See also Shunt generator Shunt wound motor Shunt jumper Zero-ohm link Fuse (electrical) References External links Electrical engineering