Los estados depredadores y la anarquía como causa facultativa

NOTE Uninterruptible power systems are covered in BS EN 62040 and batteries for use with telecom, uninterruptible power systems, utility switching, emergency power or similar applications are covered in BS EN 60896.

13.4.1

Quality of supply

a power line disturbance monitor should be connected to the power supply for a period of 1 week to 2 weeks to ensure that the supply meets the minimum requirements laid down in the equipment manufacturer’s specification. this test should be carried out

immediately after completion of the power supply installation, thus allowing the maximum period of time for any necessary modifications prior to the delivery of the computer/instrumentation equipment.

Before connecting the power supply to any item of equipment, the information on the manufacturer’s data plate on the equipment should be examined in order to ensure that the equipment is compatible with the power supply provided.

13.4.2

Standby power supply systems

Standby power supplies such as dual a.c. inverter and battery systems should be tested, where possible, with dummy loads prior to their connection to the computer/instrumentation equipment.

a power line disturbance monitor should be connected to the supply during these tests in order to ensure that the supply remains within the minimum requirements laid down in the equipment manufacturer’s specification.

13.4.3

Temporary power supplies

Where it is necessary to provide a temporary power supply prior to completion of the permanent supply, care should be taken to ensure that the temporary supply is secure and meets the system permanent power supply characteristics.

13.4.4

Continuity of supply

Whilst most process control systems are provided with power fail/auto restart facilities, it is not recommended that systems be subjected to continual and repeated power failures, as such treatment can result in the degradation of system performance. Every effort should therefore be made to maintain continuity of supply once power has been applied to a system.

NOTE 1 Earthing for instrumentation and control systems is covered in IEC 61918 and IEC 61784‑5.

NOTE 2 For further information on intrinsically safe systems, see Clause 5.

13.5

Protection

13.5.1

Smoke and fire detection

13.5.1.1 Smoke detection

Smoke detection systems, if specified, should be installed and fully tested by the supplier before the computer/instrumentation system is left powered up and unattended.

13.5.1.2 Fire detection

the fire extinguishing system should be installed and fully tested by the supplier before the computer/instrumentation system is left powered up and unattended.

Where gas flooding systems are employed, installation personnel should be made aware of safety procedures associated with such systems.

13.5.2 Surge protection

Where surge protection is specified, the devices should be fitted to the supply lines of the specified equipment only. the rating of surge protection devices should not be exceeded.

at the design stage it should be established whether surge protection is needed for equipment connected to the low‑voltage mains and for communications cables for protection against the risks from lightning strikes. SPds can be used to protect sensitive electronic equipment.

they should be connected to the LV supply lines and incoming signal/

data cables.

Surge protection devices for an installation should be selected and applied in accordance with dd CLC/tS 61643‑12 for LV systems and dd CLC/tS 61643‑22 for telecommunications and signalling networks.

at the design for installation stage, it should be established if any instruments to be installed have their own SPds. if they have then the data on them should be obtained, as these devices have to be taken into account in selecting SPds for the installation connections.

NOTE General requirements for protection against lightening are given in BS EN 62305‑1.

PD IEC/TR 62066 also gives general guidance on lightning strikes in relation to complex electrical systems, the effect on LV systems of strikes on MV systems to which they are connected, the interactions between power and communication systems arising from overvoltages and the coordination required in the selection of surge protection devices, taking into account other sources of overvoltage.

Risk management for protection against lightning is given in BS EN 62305‑2. Requirements for protection against lightning of electrical and electronic systems within structures are given in BS EN 62305‑4.

13.5.3

Electrostatic discharges

WARNING. Persons whose health depends on the correct operation of body‑worn or implanted electro‑medical devices should never be near any ESd testing or ESd testing equipment.

the following recommendations should be met for protection against ESd.

at the beginning of the installation design, the site requirements a)

for protection against ESd should be established or identified.

instrumentation to be installed should meet these requirements.

NOTE 1 PD CLC/TR 50404 gives recommendations for hazardous areas, non‑conductive solid materials, liquids, gases and powders, and precautions against static electricity on persons.

instrumentation should conform to the relevant product b)

standards for protection against ESd from personnel, when selected for the installed location. See 9.5 and 4.4.4.

additionally, consideration should be given to the installation of personnel antistatic earthing points on enclosures of sensitive instrumentation.

all instrumentation should be earthed. Before installation c)

begins, all earth bonding needed to protect against ESd should be checked. When installing in existing installations, particular care should be taken to inspect the bonding for corrosion and damage, which can be caused by vibration. Faulty bonding should be repaired before the new installation begins.

ESd from metal parts can have faster rise times than personnel d)

ESd, due to their low resistance, so can have a higher frequency spectrum than covered by the relevant product standard. also, discharges can be much greater, due to the higher capacitance of large metal objects. instrumentation can be at risk from these discharges. Where this risk exists, instrumentation should be assessed against BS En 60801‑2.

installation of instrumentation in an area with a relative humidity e)

of less than 35% should be avoided, due to the increased risk of static build‑up. if this is not possible, special precautions should be taken.

installation personnel should wear protective clothing in f)

accordance with site requirements. nylon clothing is not acceptable. Control room furniture should be ESd‑safe. Use of antistatic mats should be considered where there is likely to be a lot of personnel movement.

NOTE 2 Guidance is given in PD CLC/TR 50404.

Moving insulated cables can cause static build‑up on the g)

conductors, especially when dragging across artificial fibre carpets or vinyl floors. Before connecting to instrumentation, it should be ensured that all conductors are discharged. if soldering conductors to the instrumentation, soldering irons with an earthed tip should be used.

COMMENTARY ON 13.5.3

Electrostatic charging occurs when dissimilar materials are in rubbing or sliding contact (tribocharging) and electrostatic discharge (ESD) occurs when the materials are separated. On separation, the air becomes highly conductive when the voltage is sufficient to ionize it and a spark occurs as the separated charges try to equalize.

The size of the charge depends on the conductivity of the two materials, whether the charge between them can leak away and the position of the two materials in the tribo‑electric series: the further apart they are in the series the greater the charge. Electrostatic induction can also cause an ESD, when an electrically charged object is placed near a conductive object isolated from ground, which subsequently comes into contact with a conductive path. Discharges can also happen in a vacuum and in any insulating gasses, liquids or solids when their breakdown field strength is exceeded. Common sources of ESD are people, pulleys/

conveyor belts, paper and plastic handling, vehicles, furniture, and flowing liquids, vapours, and powders/dusts. ESD can be a significant problem in the process industries where fluids, particles, powders, plastics, paper or rubber are processed. The most common causes of ESD are moving people, low humidity (hot and dry conditions), incorrect or faulty earthing, unshielded cables, poor connections, and moving machines.

Solid‑state electrical/electronic equipment is particularly affected by ESD, particularly as miniaturization and packing density increases. DC and low frequency analogue circuits can experience momentary glitches, high frequency analogue circuits can malfunction and this can lead to erroneous data in digital circuits. Components can fail catastrophically or suffer latent damage, leading to reliability problems.

if post‑installation testing is considered necessary, it should only h)

be done with the agreement of the site operator and under a permit‑to‑work system (see 4.3), as other equipment could be affected. account should be taken of the risk of causing latent damage, especially where high reliability is required. testing should be in accordance with BS En 60801‑2.

to control EMC‑related functional safety issues, hazard and risk assessments should be carried out and should take into account at least the following issues.

What electromagnetic disturbances might the apparatus be 1)

exposed to?

What are the reasonably foreseeable effects of such disturbances 2)

on the apparatus?

how might the disturbances emitted by the apparatus under 3)

consideration affect other apparatus?

What could be the reasonably foreseeable safety implications of 4)

the above mentioned disturbances?

What level of confidence is required that the above factors have 5)

been fully considered and all necessary actions taken to achieve the desired level of safety?

these hazards and risks assessments, and the decisions that arise from them, should be treated as part of the safety validation and should be documented.

all ESd equipment used by personnel on site should conform to BS iEC 61000‑4‑2.

NOTE 3 Further guidance is given in the IET Guidance document on EMC and functional safety [90].

13.5.4

Lightning

Lightning protection units should be employed where an electronic system is connected to a data transmission line that might be subject to electrical interference, i.e. lightning. the purpose of such units is to protect the system from high voltage transients that can be induced in the data transmission line.

Lightning protection units should be installed as close as possible a)

to the equipment being protected, and strictly in accordance with the manufacturer’s recommendations.

b) Care should be taken to ensure that the equipment being protected is earthed through the lightning protection unit.

input and output cables should always be taken through separate c)

glands and segregated to avoid high voltage transients bypassing the unit.

Particular care should be taken to ensure high integrity of d)

the earthing arrangements of lightning protection units. the effectiveness of the unit is heavily dependent on compliance with the manufacturer’s earthing recommendations.

NOTE Where the equipment being protected has its electronic components insulated from its metal case then the metal case may be earthed locally.

13.5.5

Electric arc welding

Some types of interface equipment can be susceptible to damage by excessive induced voltages which can be generated by electric arc welding sets, e.g. as a result of poor earth return paths.

to eliminate the possibility of damage from this source, the interface equipment should be isolated from plant cables while any electric welding operations are in progress.