One of the most common and effective ways to install electronics in hazardous locations is using intrinsically safe protection. Intrinsically safe systems can be deployed in hazardous locations because they provide protection based on limiting both the electrical energy and thermal energy. These systems limit this energy in both normal and abnormal operating conditions so that the electronics cannot ignite a hazardous material in the materials easiest ignitable condition. Intrinsically safe circuits can be connected and disconnected even while live because of this limitation in energy. Applicable safety standards define certain electrical characteristics that must be maintained to prevent ignition. These electrical characteristics can be found in IEC 60079-11 for Europe or UL 60079-11 for the United States. These electrical characteristics include the maximum amount of energy required to ignite various hazardous mixtures, and maximum capacitance and inductance values since the stored energy can ignite hazardous mixtures.
Intrinsically safe systems must protect in both normal and abnormal operation so that even if the electrical system is in a fault condition, the system does not ignite the hazardous material. Fault conditions could include opens, shorts, and grounding of the electrical signals, as well as higher voltage levels. Different levels of intrinsically safe systems exist depending on the Zone/Division equipment that must be employed. Type “ia” provides two layers of fault prevention and can be placed into Zone 0 environments.
Designers can choose from multiple different intrinsically safe options to deploy electronic equipment. Intrinsically safe standards allow the following three options, also known as apparatus.
An associated apparatus is an electrical device typically installed in nonhazardous locations. It delivers a safe interface between measurement/control systems and field devices (sensors) located in hazardous locations. Barriers and isolators are two different types of associated apparatus.
A barrier is a passive device that consists of a Zener diode, resistor, and fuse network. The main advantages of a Zener barrier include:
- Lower part cost
- Simple device
The disadvantages of a Zener barrier include:
- Requires a dedicated safety ground
- Drop in voltage provided to field sensors
- Leakage current from the Zeners affects measurement accuracy
- Requires routine checks because of the potential of permanent damage in case of fault or incorrect installation
Another type of associated apparatus is a galvanic isolator. These devices isolate the intrinsically safe connections using transformers, optical isolators, or electro-mechanical devices. This isolator usually requires a power supply. The main advantages of a galvanic isolator are:
- Does not require a dedicated safety ground
- No voltage drop
- Better accuracy
- Grounded sensors can be used
Table 2. Comparison of Barriers and Isolators
Lower cost (~$100-250 per channel)
Dedicated safety ground required
Voltage drop to field sensors
Lower measurement accuracy
Routine checks required
Higher cost (~$130-500 per channel)
No ground requirements
No voltage drop
More robust to damage
A simple apparatus is an electrical component or combination of components with well-defined parameters that can be used in an intrinsically safe circuit. These components do not generate or store more than 1.5 V, 100 mA, and 25 mW. Examples include thermocouples, photocells, switches, junction boxes, resistors, and simple semiconductor devices.
Intrinsically Safe Electrical Apparatus
An intrinsically safe electrical apparatus is an electrical device that is designed to be directly installed in a Zone 0 or Zone 1 hazardous environment. Examples of these devices include solenoid valves and pressure transducers.