Single fault condition

The heart and the brain stem are particularly sensitive for small areas of high current density. Small area contacts occur, for example, with pacemaker electrodes, catheter electrodes, and current-carrying fluid-filled cardiac catheters. Small area contact implies a monopolar system with possible high local current densities at low current levels in the external circuit. This is called a microshock situation. The internationally accepted 50/60 Hz safety current limit for an applied part to the heart is therefore 10 pA in normal mode, and 50 pA under single fault condition (e.g., if the patient by insulation defects is in contact with mains voltage). The difference between macro- and microshock safety current levels is therefore more than three decades. 

The basis for the national or international standards (lEC, UL, VDE, MDD [the European Medical Device Directive]) is to reduce file risk of hazardous currents reaching the patient under normal conditions. Even under a single fault condition, patient safety shall be secured. 

Fault protection is protection against electric shock under single fault conditions and is provided by protective bonding and automatic disconnection of the supply (by a fuse or MCB) in aooordance with lET Regulations 411.3 to 6,... 

Matayoshi, M., Saito, M., Electrical Safety and Reliability of Health Care Facilities Equipped with Class 1 Equipment—Safety Limit for Occurrence of Single-Fault Conditions and Its Maintenance, Japanese Journal of Medical Electronics and Biological Engineering, Vol. 18, No. 2,1980, pp. 105-111. 

Protection to be provided in both normal and single fault conditions. 

The standard requires that the system operate safely in the event of a single fault condition. 

In case of an emergency, the patient has to be reachable and positionable for treatment. Thus even in a single-fault condition it has to be possible to return the device into an emergency position. 

These call for a closer proteciion, w hich is possible through a single point niolor protection relay (MPR). Since a single MPR provides protections against unfavourable operating as well as fault conditions, we discuss this relay separately in Section 12.5. 

Purpose Unfavourable operating conditions Fault conditions Protection Single-device motor protection relays Summary of total motor protection Motor protection by thermistors Monitoring of a motor s actual operating conditions Switchgears for motor protection Selection of main components Fuse-free system... 

Intrinsically Safe Will not ignite the most ignitable concentration of the hazardous material at 1.5 times the highest energy possible under normal conditions, under 1.5 times the energy of the worst single fault, and under the energy of the worst combination of two faults. Class 1, Division 1 Class 1, Zone 0 Class II, Division 1 Class III Locations... 

Machines safety circuits sometimes require special components such as relays, contactors, interlocks, and E-stops. Common terms associated with these machine components are control reliable, fault tolerant, aaA fail-safe, which means that they fail to a safe condition after a single fault (not multiple faults). 

In online FDI, the coherence vector is computed at every sampling step. If it is not a null vector, a fault is detected and an alarm is raised. Clearly, detectability is a necessary condition for a fault to be isolated. In order to simplify the task of isolating the fault, often a single fault hypothesis is adopted. It is assumed that more than one fault have not occurred simultaneously, that only one single fault may occur at a time. 

Zero-Fault Tolerant Having no redundancy. Pertaining to a condition in which a single fault in a system will cause that system or the function performed by it to fail. 

Circuit breaker coordination is a good engineering practice that can limit the degradation of safety related systems during a fault condition on a system component. Without coordination between breakers, an electrical fault on one component, such as a motor or motor-operated valve, could result in the loss of an entire motor control center or switchgear bus instead of just the single faulted component. 

Safety systems at nuclear power plants are designed with redundant trains such that a failure of a single component or train will not prevent the system from performing its safety function. Thus, a lack of breaker coordination does not necessarily pose a safety significant hazard. If a faulted condition in a safety related component causes an entire motor control center or switchgear to be lost, there is generally a completely electrically independent, redundant train to perform the same function and thus the function of the safety related system is not lost. 

For both types of requirements, the first step of the safety analysis is the computation of the minimal combinations of node faults leading to the Failure Conditions. These combinations are called Minimal Cut Sets (MCS). They are used to compute the mean probability of the Failure Condition in order to assess whether the designed architecture is safe enough. The MCS are also analysed to check whether there are combinations made of a single fault event that could lead to the Failure Condition. 

The probabilistic analysis supports the deterministic analysis by providing confidence that the safety systems used to control faulted conditions are tolerant to a single failure of an active component. The PRA also shows that the AP 1000 risks are likely to be less than UK targets, recognising that a formal demonstration is still to be presented. This forms a sound basis for the ALARP argument. 

The coverage concept was first introduced in the seminal paper by Bouricius et al. [9], also called as the coverage factor, as a conditional probability accounting for the efficiency of fault-tolerant mechanisms. If the identification and recovery of faults are independent of each other, the CM is called a single-fault model (e.g., [5, 10]) otherwise it is called a multi-fault model (e.g., [11, 12]). A recent survey on the status and trends of various CMs was presented in [3]. The issues of persistence and coverage of non-persistent components have not been addressed in these traditional CMs, in which the coverage was limited to the faulty components with a general assumption on system coherence [4, 5]. 

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