understanding intrinsic safety terminology

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Uwhat? –

Understanding

I.S. terminology

RTKtec 105

Intrinsic Safety Technical Paper

Permission to illustrate this Note with extracts from the following

publications is gratefully acknowledged:

BS EN50020

Extracts reproduced with the permission of BSI under licence

number PD\1997 1788, for training only.

It is recommended that the quoted Standard is studied in more

detail so that extracts are understood in context.



U what ? – Understand ing IS terminology

In t roduct ion

The intrinsic safety characteristics marked both on the associated apparatus and the field equipment, and quoted in therelevant certificates and manufacturer’s datasheets, need to be understood before they may be usefully applied. They employterminology and notations which are explained in this Note.

The notations which are most widely recognised are those contained in BS EN50 020 “Electrical apparatus for potentiallyexplosive atmospheres – Intrinsic safety ‘i’ ”, which is a comprehensive reference document covering almost all aspects ofthe protection technique, and which is recommended reading for all engineers concerned with IS installations.

In manufacturers’ catalogues there will also be found other voltage and current ratings which relate to normal operatingconditions and which have no direct relation to figures which define the safety parameters.

Assoc ia ted appara tus

Associated apparatus is that group of equipment which has both IS and non-IS terminals, and which performs the role ofinterfacing between the safe area and the hazardous area. It includes such items as zener barriers, isolation interfaces, etc.,together with specialised instruments such as temperature recorders and alarm annunciators which may have one or moreintrinsically safe pairs of signal input terminals.

From a safety point of view, these units will have a declaration of the maximum output which they are able to transmit to thehazardous area under specified worst conditions, which will include such considerations as the results of overvoltage at thenon-IS terminals, short circuits in the field and one or more faults simultaneously occurring within the equipment itself.

Output parameters

These maximum figures will be:-

Uo The maximum output voltage which could appear at the IS terminals under open circuit conditions.

Io The maximum output current which could flow from the IS terminals under short circuit conditions.

Po The maximum matched power which could be transmitted from the IS terminals to the equipment

in the hazardous area.

Uz As U o but usually reserved for the maximum output voltage of zener barriers, not normally used for

other types of associated apparatus.

Rmin The minimum value of the current limiting resistor in a zener barrier.

Safe ty descr ip t ion

The safety description of the output from an associated apparatus is a shorthand (sometimes only approximate) reference tothe characteristics. It usually takes the form of Uo (or Uz) with Rmin, or may be taken from Uo (or Uz) with Io. For example,

15V100R or 28V93mA.



U what ? – Understand ing IS terminology

Reactance parameters

The reactance parameters, more usually known as cable parameters or load parameters, represent the maximum values ofcapacitance and inductance which may safely be connected to the output terminals of the associated apparatus withoutinvalidating intrinsic safety.

They are:-

Co The maximum value of capacitance which may be connected to the output terminals, usually

in microforads.

Lo The maximum value of inductance which may be connected to the output terminals, usually

in millilhenries.

Lo/Ro The maximum value of distributed inductance with resistance, commonly associated with inductance

within cables, usually in microhenries per ohm.

The inductive parameters are mutually alternative, it is only necessary to satisfy one of them.

In addition to the above, there is another voltage parameter which will be specified, namely:-

Um The value of Um is that maximum voltage which may be applied to the non-intrinsically safe terminals

(safe are terminals) of the interface without compromising the IS output parameters quoted.

While 250V is a typical value, sometimes a lower value such as Um = 40V may be quoted because ofspecial considerations identified by the certifying authority.

The f ie ld equ ipment

The equipment in the field will be intrinsically safe.

This statement may at first sight appear to be obvious, but it needs to be understood. In the first place if something isintrinsically safe, this means that by its own nature, it is not able to produce any sparks or hot surfaces which might beincendive or dangerous. In the second place, many things may – although intrinsically safe in themselves – produce sparks

or get hot if too much electrical power is fed to them.

So the field equipment will either be Simple Apparatus (obviously intrinsically safe without any in-depth study) or will be morecomplex and will be certified intrinsically safe. Simple Apparatus has maximum input parameters which are only affected (insome cases) by the maximum ambient temperature in which they are operated (see RTKtec 108 – “Simple Apparatus – ornot so simple?”), while the maximum input parameters of certified field equipment are stated in the relevant certificate.

They take the form:-

Ui The maximum input voltage which may be applied to the terminals of the equipment.

Note that although the connection of the field equipment will normally draw current and thereby reducethe voltage available, the maximum input voltage is taken to be that which is available from the

i n t e r f a c e

under open circuit conditions.

Ii The maximum input current which may be permitted to flow at the terminals of the equipment.

Note that although the connection of the field equipment will normally involve a voltage drop at itsterminals, the maximum input current is taken to be that which is available from the interface whenshort-circuited.

Pi The maximum input power which the field equipment can accept.

Note that whatever the effective terminal resistance of the field equipment – matched to the outputimpedance of the interface or not – the maximum input power is equated to the matched power outputrating of the interface.

Ci The maximum value of internal capacitance which is considered to appear at the equipment terminals.

Li The maximum value of internal inductance which is considered to appear at the equipment terminals.



So far we have considered field equipment which has only one pair of terminals – terminals which are connected via aninterface to the safe area equipment.

However, quite often the field equipment which connects to the interface also has further terminals which connect separatelyto additional items in the field. As examples a temperature transmitter may connect further to a resistance thermometerelement, or an ultrasonic contents gauge will need to make an ongoing connection to a level-sensing transducer.

In such cases, the certified transmitter or contents gauge will not only have figures for maximum inputs from the interface, but

also its own maximum outputs which it might be able to deliver to the separate sensing element.

Such field equipment will therefore have both values of maximum input U i etc. at specified input terminals, and values ofmaximum output Uo etc. at other specified output terminals which connect to the sensor. In the safety context, the `input`terminals are those which receive power from the interface, although in operational terms these are actually output terminals.

Al te rna t ive no ta t ions

In older certifications, or occasionally in recent certifications by non-harmonised certifying authorities, some alternativenotations may be found. These each have equivalent meanings, some examples of which are as follows:-

Symbol Some Alternative Notations

Uo Umax Umax out Uz U__ U Voc Vmax

Io Imax Imax out Isc Ik I

Po Pmax P Wmax Wmax out

Co C Cext Ca

Lo L Lext La

Lo /Ro L/R L/Ro

Ui Umax in Vin max V

Ii Imax in Imax I

Pi Wmax in Wmax Pmax

Ci Cint Ceq. Ceff. C

Li Lint Leq. Leff. L

The suffix letters are sometimes written as normal ‘lower case’:-

Examples

Umax Ik Leq

Instead of the more usual ‘subscript’ form:-

Examples

Po Ik Wmax but the meaning is the same.

In t r ins ica l ly sa fe sys tems

Intrinsically safe systems are constructed using a knowledge of the above parameters as described in RTKtec 107 –“Assembling an Intrinsically Safe System.” The same rules, where appropriate, will apply when other types of associatedapparatus are employed.

Other parameters

Manufacturers datasheets and catalogues will contain additional figures which relate to normal working parameters, andwhich have no direct relation to the certified safety parameters described above.

These will include such notations as:-

If, Vmax, Vn, Vnom, Vwkg, Rend-to-end, etc.

Explanations of these will be given in the publications in which they appear.

RTK Engineering Ltd • St . Peter’s Square • Ha rrogate • N orth Yorkshire • HG 2 0N P • England

Telephone +44 (0)142 3 5 08 25 3 • Fax + 44 (0)142 3 5 31 13 0 • Emai l solutions@ rtkeng.co.uk

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