B.3 Steps in developing a sampling plan by applying the sampling methodology
B.3.1 Define lots
Lots of equipment should be defined by applying the following factors to the contents of an equipment register of Ex electrical equipment used in hazardous areas:
C hazardous area classification;
C type of protection;
C environmental conditions, and C age.
Therefore, equipment in a lot should be similar for one or more of the following:
C be used in the same hazardous area classification;
C have the same type of protection;
C be used in the same environmental conditions, including the same installation area, and
C have the same/similar age.
Each factor is further considered in turn in B.3.1.1-B.3.1.4. Thereafter, guidance is provided on exclusion of equipment from lots and assigning equipment to specific lots, and examples are provided.
Sample size: n
Inspect sample (see B.3.7)
Weighted faulty equipment score
< Re
> Re
Choose random sample (see B.3.6)
Inspect another sample
> 2Re
= 0
Weighted faulty equipment score (1 and
2 inspections)
Inspect complete lot or TPEF provides robust justification for
alternative action Review category of
inspection (see Figure B.3) Review ASL
Define scoring and rules for handling faulty equipment (see B.3.9) Review frequency of
inspection (see Figure B.3)
Frequency of inspection, F
Sampling plan
Review global failure rate
RBI sampling strategy to manage Ex electrical equipment ignition risks
Record inspection data (see B.3.8)
Common fault mode
Define prioritisation of repair of faulty equipment (see B.3.9.2.2)
Remedial actions.
Consider increasing level of inspection for faulty equipment (e.g.
close to detailed) (see B.3.9.2.5) Common fault mode
RBI parameters (see B.3.1-B.3.5)
Define prioritisation of remedial
Figure B.1: Flowchart illustrating steps in developing a sampling plan by applying RBI sampling strategy to manage Ex electrical equipment ignition risks
B.3.1.1 Hazardous area classification
Equipment should be divided according to the following four hazardous area classification zones for the purpose of defining lots:
C Zone 0;
C Zone 1;
C Zone 2, and C non-hazardous.
Note: In the case of offshore installations, which are compact, it might be prudent to consider external non-hazardous areas as Zone 2 areas for the purpose of defining lots and inspecting Ex electrical equipment.
B.3.1.2 Type of protection
Equipment should be divided according to the following five types of protection for the purpose of defining lots:
C type 1: flameproof (Ex ‘d’);
C type 2: increased safety (Ex ‘e’) and type 3 non incendive (Ex ‘n’);
C type 3: intrinsic safety (Ex ‘ia’, Ex ‘ib’);
C type 4: pressurised apparatus (Ex ‘p’), and
C type 5: other type of protection (oil filled (Ex ‘o’), powder filled (Ex ‘q’), encapsulated (Ex ‘m’)).
Note: As a first approach, type of protection Ex ‘n’ is grouped in type 2. If such equipment is prevalent, it could be divided into its specific types of protection (‘nA’,
‘nC’, ‘nR’, ‘nL’ and ‘nZ'); however, this has the potential disadvantage of generating very small lots for which applying the sampling methodology adds little value.
B.3.1.3 Environmental conditions
Equipment in the same installation area should be divided according to the following five environmental conditions for the purpose of defining lots:
C water, corrosion (salt, humidity, SO2, etc.), chemicals;
C dust/sand;
C ultraviolet (UV) radiation;
C ambient temperature or temperature cycling, and C mechanical damage, vibration.
Such environmental conditions have the potential to accelerate equipment ageing.
B.3.1.4 Equipment age
Equipment should be divided according to the following three age classes for the purpose of defining lots:
C ≤5 years old;
C >5 years old but ≤20 years old, and C >20 years old.
Equipment age may affect its protection; where possible, such criteria should be referenced to design life.
For simplicity and to avoid defining small lots, a pragmatic approach would be to select a single conservative age for all equipment on a particular installation, equal to the age of the installation.
B.3.1.5 Define lots according to these criteria
Ex electrical equipment should be allocated to lots using the criteria of B.3.1.1-B.3.1.4;
to facilitate this, a matrix should be defined. The number of equipment in each lot will
vary. See B.3.1.6 regarding exclusion of equipment from lots and assigning equipment to specific lots.
Note that as more of the factors in B.3.1 are applied when determining lots the equipment will tend to be more homogeneous (e.g. same hazardous area classification, same type of protection, etc); however, this has the disadvantage of resulting in small lots (see B.3.3.4.6) except where the equipment is prevalent (e.g. junction boxes, luminaries, etc.). Users should carefully consider how to best define lots based on their portfolio of equipment, its location in installation areas, typical shutdown areas, etc.
Some 80% of Ex electrical equipment in a typical installation may be located in Zone 2 hazardous areas: with such numbers, it should be beneficial to apply the sampling methodology. However, the number of Ex electrical equipment located in Zone 0 and Zone 1 hazardous areas of a typical installation may preclude application of the sampling methodology since lot sizes may be small, and more so once the additional criteria of B.3.1.2-B.3.1.4 are used to further subdivide the equipment into lots.
See B.3.3.4.6 for further guidance on applying the sampling methodology to small lots.
Following this process fully is likely to result in many small lots which is disadvantageous as it would require proportionately more inspection effort.
The simplified approach to defining lots in the absence of appropriate Ex inspection records described in section 3.2.4 and Box 3.1 can be used as a reasonable starting point; this is based upon ignition risk which is itself determined by the probability of a flammable atmosphere being present (based on duration) and the probability of a source of ignition being present (based on electrical system rated currents).
B.3.1.6 Exclusion of equipment from lots and assigning equipment to specific lots
Some circumstances may preclude equipment from being allocated to lots and including it in a sampling plan. For example, where:
C Specific grades and frequencies of inspection are required by overriding legislation, standards, etc. (e.g. for fire and gas detection equipment).
C The equipment design life is relatively short compared to the frequency of inspection such that it is unlikely to be inspected whilst in service.
Other circumstances may necessitate equipment being allocated to specific lots and including it in a sampling plan. This may apply to:
C Moveable electrical equipment (hand-held, portable and transportable), which may be used in various hazardous areas and is particularly prone to damage or misuse.
C Equipment with a particular safety integrity level (SIL) requirements.
C SCEs, which will be energised during emergencies (e.g. fire and gas detection equipment, emergency lighting, public address (PA), general alarm (GA), etc.) (see B.3.5.1).
Having such specific lots may facilitate allocation of a particular level of inspection proportionate to the additional equipment criticality. In addition, it ensures that some higher risk equipment is inspected, rather than it possibly not being randomly selected if part of a lot of mixed equipment.
B.3.1.7 Examples of lots
There are various possibilities as to how to define lots that meet the criteria of B.3.1. For example a lot could comprise mixed equipment (e.g. motors, junction boxes, luminaries, etc.) from a specific plant; defining such a lot has the advantage that in a shutdown, a sample of a mixture of equipment in that area is inspected. This also has the advantage
of not introducing small lots for which applying the sampling methodology adds little value. Other factors that may determine the subdivision of equipment into lots include the installation size, shutdown areas, etc. Note that some equipment may need to be excluded from lots or assigned to specific lots; see B.3.1.6.
Example
Lots are defined for a 15 year old offshore installation by allocating Ex electrical equipment to a matrix using the following steps:
C The installation is divided into three hazardous areas (Zone 0, Zone 1, Zone 2) and non-hazardous.
C Equipment in each sub area is divided into three types according to equipment electrical system rated currents.
This process is illustrated in Figure B.2 with equipment allocations summarised in a matrix (see Table B.1).
Zone 0 Zone 1 Zone 2
Hazardous area classification for installation:
- Zone 0 - Zone 1 - Zone 2
- offshore non-hazardous
Seven homogeneous lots defined (low, medium and high risk) -see Table B.1
Electrical system rated currents:
- intrinsically safe - instrumentation - HV/LV power circuits and emergency equipment
non-hazardous
Three hazardous areas defined and non-hazardous:
- Zone 0 - Zone 1 - Zone 2
- offshore non-hazardous
Figure B.2: Example of process for defining lots of Ex electrical equipment Note: In this example, Ex electrical equipment age and severity of the environment are the same.
Table B.1: Simplified approach to defining lots using a matrix for an offshore installation
Probability of flammable atmosphere being present Intrinsically safe Low Low
(Note 3) Low Low
Instrumentation Low Low
(Note 3) Medium N/A (Note 4)
Probability of source of ignition being present
HV/LV power circuits and emergency
equipment Medium Medium
(Note 3) High N/A (Note 4)
Notes:
1 Table B.1 is an example based on Table 3.1; clearly, the ‘onshore non-hazardous’ column is not relevant.
2 Greyed-out cells indicate where there is no such Ex electrical equipment at this installation.
3 Most of the inventory of Ex electrical equipment is located in Zone 2; in the absence of historical records, this approach provides a reasonable starting point in determining lots. However, where records exist, the next step should be to apply the full criteria for defining lots set out in B.3.1.1-B.3.1.4 for Ex electrical equipment located in Zone 2.
4 N/A – Such Ex electrical equipment is not appropriate for that zone.
5 For this offshore installation with several thousand Ex electrical equipment, seven lots would be defined: four low risk; two medium risk; and one high risk. Whereas, for a smaller installation with much less Ex electrical equipment, the four low risk lots may be grouped into one low risk lot, and similarly, the two medium risk lots may be grouped to one medium risk lot.
6 In abnormal operations a flammable atmosphere may be present in areas designated non-hazardous in normal operations; some electrical equipment may be required to be in service then such that it should be Ex electrical equipment. Offshore accommodation platforms have emergency lighting that would be expected to operate in abnormal operations.