Intrinsic safety

1. Reference legislation

• ATEX Directive: 2014/34/EU for equipment and protective systems intended for use in explosive atmospheres.
• IEC/EN 60079 standards (e.g. IEC/EN 60079-11 for intrinsic safety).

2. Input Data

• Probe Features:
• Rated voltage
• Rated current
• Rated power
• Inductance
• Capacity
• ATEX Zone:
• Zone 0, 1 or 2.
• Type of gas or dust (e.g. IIC, IIB).
• Ambient and maximum temperature.
• Intrinsic barrier:
• Output voltage
• Output current
• Output inductance
• Output capacity
• Maximum power transferred.

3. Essential Calculations

• Compatibility between Probe and Barrier:
• Verify that the probe values ​​are less than or equal to the barrier values:
• Ui≤Uo,Ii≤Io,Pi≤PoU_i \leq U_o, \quad I_i \leq I_o, \quad P_i \leq P_oUi​≤Uo​,Ii​≤Io​,Pi​≤Po​
• Maximum Transferable Energy Calculation:
• Determination of stored energy:
• E=12⋅Co⋅Uo2 12⋅Lo⋅Io2E = \frac{1}{2} \cdot C_o \cdot U_o^2 \frac{1}{2} \cdot L_o \cdot I_o^2E=21​⋅Co​⋅Uo2​ 21​⋅Lo​⋅Io2​
• Compatibility with Explosive Atmospheres:
• Compare the barrier protection level with the gas/dust group and the ignition temperature.
• Checking Safety Limits:
• Comparison between:
• L_i L_cable ≤ L_o.
• C_i C_cable ≤ C_o.

4. Safety Margin

Make sure there is an adequate safety margin for each parameter, also considering the length and type of cables.

5. Documentation

• Complete technical specification of the probe and barrier.
• ATEX Declaration of Conformity.
• Installation diagrams.

There probability of failure on demand (PFD) is a key measure in functional safety, specifically defined in EN 61508 standard Part 6 (Ed. 2). Quantifies the probability that a safety function will fail to perform the required action on demand and is typically used to evaluate the reliability of safety systems.

To calculate the PFD for a system according to EN 61508 Part 6, several are needed input parameters key . These parameters help define the reliability of the system and the level of security it can provide.

Key input parameters for PFD calculation according to EN 61508 Part 6, Ed. 2:

Safety Integrity Level (SIL):

SIL 1, SIL 2, SIL 3 or SIL 4

The SIL is typically determined based on the evaluation of the risk (or risk matrix ) of the system.

Component failure rate (λ):

λ (Failure Rate):

Failure rates are often based on reliability data provided by manufacturers or specific industry standards such as IEC 61508 , IEC 61511 or FIT (Failures in Time).

Test interval (T):

T (Test interval): The time between tests or inspections of safety functions. This value is important for calculating the PFD, as it takes into account the frequency of testing or maintenance of the system.

The shorter the test interval, the lower the PFD, as testing can help identify failures before a demand occurs.

MTTR:

DC (Diagnostic Coverage):

CCF

System Configuration

Test interval (PTI):

The PFD for a system can be calculated using the following general formula:

PFD=λ⋅T+(1−DC)⋅MTTR2PFD = \frac{\lambda \cdot T + (1 - DC) \cdot \text{MTTR}}{2}PFD=2λ⋅T+(1−DC)⋅MTTR

The EN 61508 standard defines the following acceptable PFD ranges for each Safety Integrity Level (SIL):

SIL 1 : PFD = 0.1 to 0.01

SIL 2

This value would be acceptable for SIL 2 as it falls within the acceptable range between between 0.01 and 0.001 .

Conclusion

To calculate the PFD second EN 61508 Part 6 , it is necessary to carefully evaluate the system failure rates, test intervals, diagnostic coverage, repair times and the overall system architecture. These parameters directly influence the safety integrity and the level of risk reduction provided by the safety function.

To calculate the PFH (Probability of Failure per Hour) According to EN 61508 Part 6, Ed. 2 using the data you provided, we can use a similar approach to the PFD calculation, but applied to obtain PFH values. This calculation focuses on the probability of failure per unit time, which is the key metric for the safety of a system.

Input Data:

• Failure rate for dangerous undetected errors (λ): 357.2 FIT
• Failure rate for dangerous errors detected (λ): 1770 FIT
• Regular test interval: 1.5 years
• Repair time for detected errors: 78 hours
• Repair time for undetected errors: 8 hours
• Percentage of undetected errors with common cause: 15%
• Percentage of errors detected with common cause: 2%

Calculating PFH:

The formula for calculating the PFH for a system in accordance with EN 61508 is as follows:

PFH=(λ⋅T)+(1−DC)⋅MTTRTestTime(inhours)PFH = \frac{(λ \cdot T) + (1 - DC) \cdot MTTR}{Test Time (in hours)}PFH=TestTime(inhours)(λ⋅T)+(1−DC)⋅MTTR​

Where:

λ is the failure rate per unit time (in FIT)

T is the time interval for the test (in hours)

DC is the diagnostic coverage

MTTR is the Mean Time To Repair in hours

Formula for different systems:

PFH for Single Channel System (1oo1):

PFH1oo1=λ⋅T+(1−DC)⋅MTTR2PFH_{1oo1} = \frac{λ \cdot T + (1 - DC) \cdot MTTR}{2}PFH1oo1​=2λ⋅T+(1−DC)⋅MTTR​

PFH for 2oo2 System:

PFH2oo2=2⋅PFH1oo1⋅(1−PFH1oo1)PFH_{2oo2} = 2 \cdot PFH_{1oo1} \cdot (1 - PFH_{1oo1})PFH2oo2​=2⋅PFH1oo1​⋅(1−PFH1oo1​)

PFH for 1oo2 System:

PFH1oo2=PFH1oo1⋅(1−PFH1oo1)PFH_{1oo2} = PFH_{1oo1} \cdot (1 - PFH_{1oo1})PFH1oo2​=PFH1oo1​⋅(1−PFH1oo1​)

PFH for 1oo3 System:

PFH1oo3=PFH1oo1⋅(1−PFH1oo1)⋅(1−PFH1oo1)PFH_{1oo3} = PFH_{1oo1} \cdot (1 - PFH_{1oo1}) \cdot (1 - PFH_{1oo1})PFH1oo3​=PFH1oo1​⋅(1−PFH1oo1​)⋅(1−PFH1oo1​)

PFH for the 2oo3 System:

PFH2oo3=3⋅PFH1oo1⋅(1−PFH1oo1)⋅(1−PFH1oo1)PFH_{2oo3} = 3 \cdot PFH_{1oo1} \cdot (1 - PFH_{1oo1}) \cdot (1 - PFH_{1oo1})PFH2oo3​=3⋅PFH1oo1​⋅(1−PFH1oo1​)⋅(1−PFH1oo1​)