Galvanic isolation (also referred to as galvanic separation or electrical isolation) is the general principle of electrically isolating a given circuit from other circuits within the same device. It refers to the transfer of energy or signals between electrical circuits without direct electrical contact. Galvanic isolation is used for signal transmission, contactless control, and for protecting equipment and personnel from electric shock.
An example of galvanic isolation is a transformer. The primary winding of a transformer is completely isolated from the secondary winding, meaning no current can flow between them (except in the event of insulation breakdown), even if there is a high potential difference between the windings. Thus, even if the secondary winding is galvanically connected to the chassis and, consequently, to ground, no parasitic currents dangerous to equipment or personnel will occur on the chassis.
Galvanic isolation of circuits can be implemented by various technical means: transformer (inductive) isolation (transformers, digital isolators based on high-frequency transformer principles), optical isolation (opto-couplers, opto-relays), capacitive isolation (digital isolators based on capacitive coupling), and electromechanical isolation (electromechanical relays). An opto-coupler, capacitor, or transformer allows electrical signals to be transmitted without electrical contact. Isolated sections of a circuit interact through an electrostatic field in the case of a capacitor, a magnetic field in the case of a transformer, and light emission in the case of an opto-coupler.
The opposite concept is galvanic connection, which applies when there is a direct electrical connection between two or more sections of a circuit. Galvanic isolation, conversely, is the organization of circuit interaction in such a way that no direct electrical contact is present.
Galvanic isolation is used to address two primary tasks:
1. Ensuring the independence of a signal circuit (when connecting instruments and devices) by providing a separate current loop for the signal circuit relative to other current loops that occur when devices are interconnected. For example, this may involve isolating a measurement circuit from a power control circuit. Independence of the signal circuit addresses numerous electromagnetic compatibility (EMC) issues: it improves noise immunity, signal-to-noise ratio in the signal path, and measurement accuracy. A galvanically isolated input or output always improves compatibility with other equipment in challenging electromagnetic environments. In multichannel measurement or data acquisition systems, galvanic isolation may be implemented as group isolation (one isolation barrier for several channels) or channel-by-channel isolation (individual isolation for each measurement channel).
2. Ensuring electrical safety when working with equipment in compliance with national and international safety standards. For electrical equipment intended for measurement, control, and laboratory use, the standard GOST 52319-2005 specifies requirements for insulation strength (test voltage). It should be noted that galvanic isolation is one of several technical measures to ensure electrical safety; therefore, insulation requirements for a specific circuit must be considered together with other protective measures (protective grounding, current and voltage limiting circuits, etc.) implemented in accordance with GOST 52319-2005.
Disadvantages of galvanic isolation
The main disadvantage of galvanically isolated circuits is the increased noise level from DC/DC converters. For low-frequency circuits, this noise can be sufficiently reduced by digital and analog filtering. At high frequencies, the capacitance to ground and the capacitance between transformer windings limit the effectiveness of galvanic isolation. Ground capacitance can be reduced by using optical fiber and minimizing the physical dimensions of the isolated subsystem.
A common mistake when using galvanically isolated circuits is the incorrect interpretation of the term “isolation voltage.” For example, if the isolation voltage of an input module is specified as 3 kV, this does not mean that its inputs can operate continuously under such high voltage in normal conditions.
In foreign literature, three standards are commonly referenced: UL 1577, VDE 0884, and IEC 61010-01. However, device descriptions do not always cite these standards, leading to ambiguous interpretation of the term “isolation voltage” in domestic documentation of imported equipment.
The main difference lies in whether the voltage refers to the continuous working isolation voltage (applied indefinitely) or to the test isolation voltage (applied for a short time, from one minute to a few microseconds). Test voltage can be up to 10 times higher than the working voltage and is intended for accelerated testing during manufacturing, as its effect on insulation depends on the test pulse duration.
Relationship between working and test isolation voltages according to IEC 61010-01
| Working voltage, V | Air clearance, mm | Test voltage, V | ||
| Peak impulse voltage, 50 xs | RMS value, 50/60 Hz, 1 min | DC voltage or peak AC value, 50/60 Hz, max., 1 min | ||
| 150 | 1.6 | 2550 | 1400 | 1950 |
| 300 | 3.3 | 4250 | 2300 | 3250 |
| 600 | 6.5 | 6800 | 3700 | 5250 |
| 1000 | U5 | 10200 | 5550 | 7850 |
The table shows the relationship between working and test isolation voltages according to IEC 61010-01. As can be seen, working voltage, DC, RMS AC, and peak AC test voltages can differ significantly.
The dielectric strength of insulation in domestic automation equipment is tested according to GOST 51350 or GOST R IEC 60950-2002, i.e., using sinusoidal voltage at 50 Hz for one minute at the test voltage specified in the operating manual. For example, with a test isolation voltage of 2300 V, the working isolation voltage may be only 300 V (see Table 1).
Related sections:
Atmospheric interference,
Industrial interference,
Multipath interference.