H.27.1 Electronic controls – Assessment against internal faults
H.27.1.1.5 Electronic circuit fault conditions
For the purpose of Clause H.27, the applicable failure modes are given in Table H.24.
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Table H.24 (H.27.1 of edition 3) – Electrical/electronic component fault modes table (1 of 3)
Component type Short a Open b Remarks
Fixed resistors Thin-film c Thick-film c
Wire-wound c (single layer) enamelled or suitably coated
All other types X
X X X
X
Includes SMD type Includes SMD type
Variable resistors
(for example, potentiometer/trimmer) Wire-wound (single layer)
All other types X d
X X Capacitors
X1 and Y types according to IEC 60384-14 Metallized film according to IEC 60384-16 and IEC 60384-17
All other types X
X X X Inductors
Wire-wound
All other types X
X X Diodes
All types X X
Semiconductor type devices like transistors All types (for example, bipolar; LF; RF;
microwave; FET; thyristor; Diac; Triac; Uni
junction) X d X e
Hybrid circuit f f
Integrated circuits
All types not covered by H.11.12 X g X For IC outputs, footnote e
applies Optocouplers
According to IEC 60335-1 X h X
Relays Coils
Contacts X i
X X
Reed-relays X X Contacts only
Transformers
According to IEC 61558-2-6 or IEC 61558-2-16
All other types X d
X X
Crystals X X j
Switches X X k
Connections (jumper wire) X l
Cable and wiring X
Table H.24 (2 of 3)
Component type Short a Open b Remarks
Printed circuit board conductors
According to IEC 62326 series X m X n
Sensors
Polymeric type thermistors Ceramic type thermistors
X X
X
o
a The conditions which have led to the design of the clearances and creepage distances according to Clause 20 on the assembly for which exclusion from the fault mode “short” is claimed shall be maintained over the lifetime of the control.
These conditions shall be declared or documented as follows.
Control pollution situation (Table 1, requirement 49).
Pollution situation in the micro-environment of the creepage distance or clearance, if cleaner than that of the control, and how this is designed (documentation) (Table 1, requirement 79).
Rated impulse voltage of the control (Table 1, requirement 75).
Rated impulse voltage for the creepage distance or clearance, if different from that of the control, and how this is ensured (documentation) (Table 1, requirement 80).
The values designed for tolerances of distances for which the exclusion from fault mode "short" is claimed.
(declaration and documentation) (Table 1, requirement 81).
b Only opening of one pin at any one time.
c These components may be used for protective impedance, if the impedance of components complies with H.20.1.15.3 and withstands the impulse voltage test of 20.1.12 for at least overvoltage category III.
d Short-circuit each pin in turn with every other pin; only two pins at a time.
e For discrete or integrated thyristor type devices such as Triacs and SCRs, fault conditions shall include short circuit of any terminals with the third terminal open-circuited. The effect of any full wave type of component, such as a Triac going into a half-wave condition, either controlled or uncontrolled (thyristor or diode, respectively) shall be considered.
Failure of a field effect based electronic power switching device (FET, MOSFET, IGBT) by loss of gate (base) control* resulting in a partial turn-on mode, causing an undefined state shall be considered.
Testing and assessment criteria shall correspond to the specific control function and circuitry. Guidance might be given in parts 2.
*Loss of gate control might occur due to, for example, an insufficient solder connection of the FET.
f Failure modes for individual components of the hybrid circuit are applicable as described for the individual components in this table.
g The short circuit of any two adjacent terminals and the short circuiting of - each terminal to the IC-supply, when applicable at the IC;
- each terminal to the IC-ground, when applicable at the IC.
The number of tests implied for integrated circuits may normally make it impracticable to apply all the relevant fault conditions or to assess the likely hazards from an appraisal of the circuit diagram of the integrated circuit.
It is therefore permissible first to analyse in detail all the possible mechanical, thermal and electrical faults which may develop either in the control itself or its output, due to the malfunction of the electronic devices or other circuit components, separately or in any combination.
Except for types evaluated by H.11.12, a fault-tree analysis shall be conducted to include the results of multiple steady-state conditions to outputs and programmed bi-directional terminals for the purpose of identifying additional fault conditions for consideration. The failure mode "short circuit" is excluded between isolated sections for such ICs that have isolated sections. The isolation between the sections shall comply with the requirements of 13.2 for functional insulation.
h When optocouplers comply with 20.3.2.2, the shorting between the input and output pins is not considered. ! "
Table H.24 (3 of 3)
I The short-circuit mode is excluded for relays where the contact is controlling a Class A or B control function, and is not included in the specified cyclic operation of the application, provided that it is successfully tested to Clause 17 under the following conditions:
For Class A control functions, the number of cycles declared by the manufacturer, or certified for the application.
For Class B control functions, a minimum of 100 000 cycles or otherwise specified by the manufacturer, whichever is higher, or certified for the application.
For Class C control functions, the short-circuit mode is excluded for relays where
• the contact function is switching under normal operation without load (no current flowing during make or break) and
• the contact function is disconnecting the load under abnormal operation of the burner control system to reach safety shut-down and
• the relay complies with Clause 17 at a minimum of 100 000 cycles however applying the declared output load, or certified for the application, and
• the control complies with the requirements of 11.3.5.2.1 (measures to prevent common cause errors).
j For crystal-based clocks, harmonic and sub-harmonic frequency variations affecting the timings should be considered.
k If switches are applied for the selection of safety times, purge times, programmes and/or other safety- related settings, these devices should function so that in the event of their opening, the safest possible condition arises (for example, in a burner control system, the shortest safety time or the longest purge time).
The short-circuit mode is excluded for switches where the contact is controlling a Class A or B control function, provided that it is successfully tested to Clause 17 under the following conditions:
For Class A control functions, the number of cycles declared by the manufacturer, or certified for the application.
For Class B control functions, a minimum of 6 000 cycles for manual action, or 100 000 cycles for automatic action, or otherwise specified by the end product standard, or certified for the application.
For Class C control functions, short-circuit mode is not excluded.
l The requirements are the same as footnote n, except they are applied to jumper wires intended for clipping when selecting a setting.
m The short-circuit failure mode is excluded if the requirements of Clause 20 are fulfilled.
n The open-circuit failure mode, i.e. interruption of a conductor, is excluded if
• the thickness of the conductor is equal to, or greater than, a nominal value of 35 àm with a tolerance up to –30 % of the nominal value allowed, and the breadth of the conductor is equal to, or greater than, a nominal value of 0,3 mm with a tolerance up to –30 % of the nominal value allowed, or
• the conductor has an additional precaution against interruption, for example, roll-tinned, etc.
If a short circuit at the output terminals causes the opening of a printed circuit board conductor, that conductor shall be subject to an open-circuit fault analysis.
For temperature and current conditions when accepting conductor sizes, see IEC 62326 series.
o For sensor components intended to measure activating quantities such as temperature, pressure etc., fault modes in addition to open and short must be considered. These fault modes can include step wise shift of resistance, non-responsive component, and component drifting out of accuracy.
H.27.1.1.6 If the load includes a motor load (see 6.2.2 or 6.2.5), and the failure or malfunction of an electronic circuit component causes a change in the supply waveform to the controlled motor, the control shall be subjected to the following tests.
1) The load shall be adjusted under normal waveform conditions to six times the rated load or the locked rotor rating declared by the manufacturer.
2) Then the fault conditions shall be introduced.
3) The test is conducted under the conditions described in items a), c), d) and e) of H.27.1.1.2.
The control shall be evaluated according to items a) to e) inclusive of H.27.1.1.3, as appropriate to the component being assessed.
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