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What is the use instrument barriers? Can you give me review materials about this?
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Answer Posted / anzal
I.S. BARRIER INTERCONNECTION
As illustrated in Figure 5-1, an intrin-sic safety barrier is inserted in the non-hazardous, or safe area between the instrument and the field device. The bar-rier
blocks dangerous energy from being transmitted from the instrument to the hazardous area. This energy could be from a power supply, stored in capacitors,
stored in inductors, or some combination of the three. This energy could be released due to some combination of faults (open circuits, shorts, grounds, etc.) occurring in the system. The field devices used in any hazard-ous area must be one of two types:
Simple Apparatus - Barriers may be used with devices which qualify as “simple apparatus” without specific approval for the simple apparatus. A simple apparatus is a field device which meets the following requirements:
u Device does not store more than 1.2 Volts or 20Joules.
u Device does not draw more than 100 mAmps.
u Device does not dissipate more than 25 mWatts.
Some examples of simple apparatus include: thermocouples, RTD’s, switches and LED’s.
I.S. Certified Apparatus -Any device which does not fall into the category of simple apparatus must be I.S. Certified. This includes transmitters, current to pres-sure convertors, solenoid valves, et. All Certification may come from FM, CSA, BASEEFA, or the regional qualifying agency (see chapter 8 for more details).
An intrinsic safety barrier is used to connect a non-certified piece of electrical equipment in a safe area to a certified or simple field device in a hazardous area. A barrier cannot be used to make an uncer-tified device safe in a hazardous area. If a field device is uncertified, it can have inter-nal energy storing components. These com-ponents, in the case of a fault, may cause a spark, which in turn may start the com-bustion process. The barrier only protects the non-certified device in the safe area from transmitting dangerous energy to the hazardous area. In summary, the barrier is an energy limiting device placed on the electrical wires between the safe and haz-ardous areas.
There are two basic varieties of intrinsic safety barriers: the zener barrier and the
active barrier. Let’s review each of them in a little detail.
ZENER BARRIER
The zener diode barrier works by di-verting potentially dangerous energy to ground before it can reach the hazardous area.
The zener diodes limit the fault voltage to the hazardous area. There are two such diodes for redundancy. The series resistor limits current to the hazardous area, and is considered an infallible component. The arrow in Figure 5-2 shows the resultant path if excess current enters the barrier as a result of excess voltage input from the in-strument. Since a zener barrier is powered by the loop and it has a current limiting resistor, it has a voltage drop across it. This bar-rier, in addition to protecting against dangerous energy entering the hazardous area, must allow the loop to function normally.
The example in Figure 5-3 shows a 24 V, 4-20 mA transmitter loop. In this example, the sum of the voltage drops around the loop can be calculated and verified that they are less than 24 V. Instead, since the maximum resistance of the barrier is specified (RMAX END TO END), Ohm’s law is used (V = IR) to calculate the total resistance available in the loop. Either method (loop voltage or loop resis-tance) works identically. What we are try-ing to decide is whether or not the resis-tance of the barrier will adversely affect the loop.
ACTIVE BARRIER
An active barrier uses transformers, opto-isolators or relays to provide isolation between the hazardous area and the non-hazardous area. It does not require an in-trinsically safe ground connection. Either or both the hazardous area or the non-haz-ardous area signals may be grounded. This may be the most logical barrier choice if a high quality I.S. grounding point is not available.
Figure 5-4 shows a diagram of an active barrier. The barrier requires a 24 Vdc power supply, and can typically drive larger loads than a loop powered zener barrier.
REQUIREMENTS FOR
INTRINSIC SAFETY
Intrinsically safe systems should satisfy these requirements: u Ensure that there is a positive sepa-ration of intrinsically safe and non-intrin-sically safe circuits. This prevents the ignition capable energy from intruding into the intrinsically safe circuit.
u Separate conduit, panduit, cable trays, etc. should be used for I.S. wiring to keep it separate from the non-I.S. wiring. If conduit is used, it must be sealed per
the applicable code. This is to prevent the conduit from becoming a means of conduct-ing flammable material from the hazard-ous to the non-hazardous location. I.S.wiring must always be identified. This can be performed by tagging the wires, label-ling the junction boxes, and/or color-coding the I.S. wires light blue.
u Ensure that the entity parameters, upon which approval of the system is based, match up correctly.
ENTITY CONCEPT
The entity concept has been approved by Factory Mutual since 1978. The approval agency specifies the maximum energy a barrier can ever deliver, as well as the maximum energy a field device can ever receive and still be safe. The entity concept allows interconnection of I.S. barriers to I.S. field devices not specifically tested in such combinations. It also allows mixing of equipment from different manufacturers without additional approvals.
Certification of barriers and field devices is based on the following conditions. Barrier entity parameters (from FM) must state the maximum voltage and current delivered under fault conditions. A field device entity parameter must state the maximum current and voltage which the hazardous area device can safely receive.
An I.S. barrier can safely be used with a field device if it has a maximum voltage and current less than or equal to that which the field device can safely receive. All these parameters are defined by the approval agency.
In addition to voltage and current, the entity parameters also include two more terms to ensure that the energy storing devices in the field do not store up a dangerous amount of energy. These terms deal with the total capacitance and total inductance of the system.
A barrier has parameters which state the maximum capacitance and maximum inductance which can be safely connected to it. A field device has parameters which state the equivalent capacitance and in-ductance of the field device. Since wiring has capacitance and inductance, this also needs to be considered. The capacitance of the field wiring must be added to the capacitance of the field device to deter-mine whether the total capacitance for the field exceeds the maximum allowed for connecting to the I.S. barrier. The same principle applies to the inductance.
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