Specific Requirements of Instrument Cabinets

Temperature:

The temperatures to which I&C equipment may be exposed in the application shall be clearly identified. The temperatures of concern shall be evaluated against the specified operational temperature requirements for the selected equipment to ensure compatibility. If equipment selection is not conducive to the given temperature conditions, alternate measures shall be taken, such as the use of the temperaturecontrolled enclosures.

Guidance: The temperature of concern is the temperature of the medium (whether air or liquid) which affects or cools the equipment. In regard to fan-cooled equipment, the temperature of concern is that of the air entering the equipment. Operational temperature requirements for equipment are normally well defined in the manufacturer’s literature. Two separate temperature ranges are typically specified, one for when the equipment is in operation and another for when the equipment is powered-down, shipped, or in storage. Operating temperatures may also be specified as ambient, which refers to the surrounding temperature, and process, which refers to the process media being measured. The manufacturer’s equipment specifications may also include a maximum allowable rate of change of temperature, given in degrees per hour.

Airflow:

The design and control of airflow systems shall consider both equipment locations and normal airflow patterns.

Guidance: Airflow in fan-cooled and convection-cooled equipment is generally vertical through the enclosure and can be from either the bottom or top. For rooms containing equipment with downward airflow, the air supply should be overhead and the return plenum should be low or in the floor. If a raised floor is in place, the space under the floor may provide the return plenum. For upward airflow, the use of the sub floor space as a supply plenum should consider the additional design considerations and continuing maintenance to prevent the infiltration and accumulation of dust, dirt, and moisture under the floor.

Relative Humidity:

The selection of equipment shall consider the relative humidity to which I&C equipment may be exposed in the application. If necessary, the design shall incorporate the use of humidity control equipment to assure operation within the defined limits for the selected equipment.

Guidance: The operating relative humidity requirement for equipment is normally well defined in the manufacturer’s literature and typically given as an operating range and a maximum time rate of change. Limitations may be given for shipping and storage as well as for operation. Typically, the desired operating range is about 40 – 60 percent. Low relative humidity (less than 30 – 40 percent) can result in system errors or shutdowns due to generation of static electricity. At LANL, this is addressed with proper grounding rather than humidification. High relative humidity can lead to condensation.

Particulate Contamination:

The presence of particulate matter (dust or dirt) shall be considered for its effect on I&C equipment.

Guidance: Dust, grit, and sand present at the inlet of process media sensing devices can prevent the equipment from performing its function. Dust build-up decreases the ability of electrical components to shed their heat, which decreases longevity. In fan-cooled equipment, the accumulation of dust on filter media will reduce airflow and cause overheating. If the dust is conductive, it can cause faults: if nonconductive, it can infiltrate and insulate switches and contacts. Careful, meticulous sealing of all equipment enclosure openings will reduce contaminant infiltration.

Chemical Contamination:

Consideration shall be given to potential chemical contamination and corrective action shall be taken to limit any potential contamination below levels that could adversely affect equipment performance.

Guidance: Certain chemicals, including sulfur dioxide, oxides of nitrogen, hydrogen sulfide, and ammonia, are known to affect electronic equipment at concentrations safe for human occupancy. Most corrosion processes accelerate rapidly at increased temperatures or humidity level (or both). Some maximum allowable levels recommended by equipment manufacturers are below levels that can be readily measured.

Vibration and Shock:

The proposed location of I&C equipment shall be evaluated for potential sources of vibration and shock, such as nearby heavy rotating or stamping equipment or heavy mobile traffic. Consideration shall be given to potential vibration and shock sources when mounting I&C equipment to assure operation within the equipment manufacturer’s defined limits.

Guidance: Continuous vibration can cause slow degradation of contacts and any mechanical parts. Shock can instantaneously change an instrument adjustment, as well as cause effects similar to vibration. It is usually more practical to relocate equipment or to apply controls at the vibrating equipment than to try to isolate the equipment from the vibration.

Power Line Conditioning and Backup:

The equipment manufacturer’s power requirements shall be met. In many cases, meeting these requirements involves more than just supplying the appropriate voltage and ampacity ratings. Frequently a special type of receptacle is required, which is usually well defined in the manufacturer’s literature. Transient Suppressors may be required depending on the type of device. Tolerance to voltage transients and brownouts are also typically defined in the manufacturer’s literature. ANSI standards permit user line voltage to be as much as 11.7 percent below nominal. Brownouts may cause additional voltage reductions of 3 to 10 percent. These reductions may severely disrupt equipment operations and may necessitate the need for power conditioning and/or backup power supplies.

Guidance: Certain critical systems should be able to operate through a power dip or an extended power outage; these should be provided with a backup power supply. For less critical systems, a packaged power conditioning system should be considered.

Electromagnetic Interference (EMI):

The proposed location of I&C equipment shall be evaluated for potential sources of EMI and consideration shall be given to its effect on the operation of the equipment. EMI results from electromagnetic emissions generated by and coupled to equipment or systems (or both).

Guidance: Common EMI sources include thunderstorms, high voltage power lines, power tools and manufacturing machines, relays, contactors, motors, vehicle ignitions, and arc welders. Isolation, shielding, and grounding may be required to prevent expected problems.

Radio Frequency Interference (RFI):

The proposed location of I&C equipment shall be evaluated for potential sources of RFI and consideration shall be given to its effect on the operation of the equipment. RFI results from electromagnetic fields generated by communication and electronic equipment.

Guidance: Common RFI sources include hand held radio transmitters, cell phones, proximity to radio or television disks or towers, and proximity to communication relay disks or towers. Generally, RF fields within the facility should not exceed 0.5 v/m. Not more than 1V RMS, in the frequency range of 10kHz to 3 MHz, should exist on the ac connection points to the system. Isolation, shielding, and grounding may be required to prevent expected problems.

Static Electricity:

The potential for static electricity problems shall be determined and if present, prevented or corrected.

Guidance: Static electricity can have a significant effect on digital equipment and equipment connected to explosive applications or in explosive environments. The catastrophic effect is the breakdown and permanent damage of semiconductor devices. The transient effect is the introduction of extraneous logic signals or voltages induced on ground or signal wiring, which can result in operational error.

1 Like