Nuisance trip

The setting may be kept on the higher side to avoid nuisance tripping due to ... 

Many years later in Singapore, we were using a specially formulated thermally conductive glue to fix the overtemperature sensing thermistor smack on to the very plastic body of the TO-220 power transistor. We had empirically ascertained that in this way, the junction temperature and the adjacent temperature as seen by the thermistor were less than 10°C apart, even during an abnormal event. So if, for example, we wanted to have the transistor turned off just before it hit 150°C, we simply needed to set the trip temperature (of the thermistor-based circuit) at about 140°C. In that way, we could also be sure that we wouldn t encounter nuisance tripping on a particularly hot day, when the temperature inside the enclosure would also be much higher. 

Ground fault protection, including circuit breakers or relays, should be set as low as possible without causing nuisance tripping. The ground fault pickup and time delay for the main overcurrent protection should coordinate with... 

It is important to define how a SIS is going to be maintained while the process is running. For example, if a transmitter or valve needs to be worked on, consideration needs to be given on how the maintenance department will work on these instruments without causing a nuisance trip while maintaining the safety of the process. 

Alarm and shutdown set points will not be set out of range or disconneeted to avoid nuisance trips or other... 

Based on the system as shown in the figure above, draw a fault tree to represent a nuisance trip due to the failure of TXl / 2 subsystem, i.e. the top event to be "Nuisance trip due to failure of transmitter subsystem." Transmitter voting is loo2. The following basic events are to be used ... 

Calculate the probability of a nuisance trip using the following data ... 

For a three-year test interval the PFDavg is 1.20E-03. This provides a Risk Reduction of 82 which no longer meets the requirements of S1L2. A spurious or nuisance trip is likely to occur every 11.1 years. 

The above analysis shows the importance of clearly defining SIF outputs. Of the three cases, case two provides the best safety performance but the worst nuisance trip rate. 

Fuses must be properly rated for performance in respect to voltage, current characteristics, and breaking capacity, and they must comply with the European standards. Using fuses and fuseholders that are European approved ensures conformity with the standards. A time-delay fiise, type T, may be used to avoid nuisance tripping, but the delay may not result in a fire or overheat condition during a fault test. Select the lowest value possible according to the actual current (not calculated current). 

On the electrical side it was determined that that NO trip out condition should be entered into. All scenarios were Risk Assessed and a work shopped. To avoid unnecessary and/or nuisance trip outs it was suggested that the following safety features be bridged out while slinging the new drum shaft for the Sub Vertical Rock winder using the Surface Man Winder. 

An earth leakage test measures the leakage current to earth through the insulation. It is a useful test to ensure that the portable appliance is not deteriorating and liable to become unsafe. It also ensures that the tested appliances are not responsible for nuisance tripping of RCDs (RCDs - see Chapter 3). A satisfactory reading is typically less than 3mA. 

Now set the test instrument to 50% of the rated tripping current of the RCD and press the test button. The RCD should not trip within 2 s. This test is testing the RCD for inconvenience or nuisance tripping. 

Operators reported excessive number of nuisance trips. 

This could worsen the Safety Culture if spurious trip rate is excessive, causing personnel to view these as nuisance trips. 

Easy conversion of a system to other systems is possible, for example, reliability block diagram, and system model for evaluation of nuisance trip rates. 

In the case of de-energize type logic set of contacts, opens to trip or shutdown the system (in case of energize to trip logic, contact closure will be considered for trip also series, parallel configuration types will be reversed). When trip is associated with proper cause it is safe failure. When this is NOT, then it is spurious/nuisance trip or shutdown. As per lEC 61508 Demand or failing element commands output to a safe state. 

The economic impact of a spurious or nuisance trip of an ESD system can be disastrous. An ESD system is an important layer of protection to prevent and prevent hazardous situations from occurring. So, it is needless to mention that the ESD system must be extremely reliable and function on demand. During an emergency, it must put the process in a safe state in orderly fashion. Also ESD systems design shall be based on a fail safe independent system, that is, ESD systems are such that during a failure of a component the process reverts to a condition considered safe and not a vulnerable serious hazardous event. Reliability and availability are major parameters for ESD system performance. Reliability is a function of system failure rate (its reciprocal) and mean time between failures. Spurious trip conditions may initiate a so-called fail safe incident that may result in accidental shutdown of equipment or processes. However, undetected process design errors or operations may initiate dangerous incidents that may disable the safety interlock and may even cause accidental process... 

Trip on demand only Reduction in nuisance trips Device self diagnostics Multi-drop architecture Installation flexibility Asset management tools Cost reduction... 

FF-SIF provides better diagnostics to minimize nuisance trips and offers potential to integrate directly into standard control architectures with improved asset and event management and lesser chances of failure. 

SIL assignment No nuisance trip Diagnostics and online testing Redundancies Layer of protection analysis Reducing PFD Common mode assessment O M procedure... 

FIGURE 5.136 Graph illustrating the dangers of adjusting an overload relay above the full-load current of the motor to prevent nuisance tripping on start-up. 

The solid-state overload relay, by contrast, will trip within seconds under the same conditions. This provides enough of a delay to avoid nuisance tripping in momentary outages, but eliminates the potential for extended operation in phase loss condition under load. 

Section 11.7 recommends that temporary decorative loads such as Christmas tree lights should be supplied at SELV. In some cases, this compromises the attractiveness of the decorations and there is a temptation to supply them at 240 V to achieve the desired brilliance and luminance. In that case, if the lighting fixtures can be accessed by the public, there will be a need to provide RCD protection to minimise the risk of electric shock injuries, although the potential problems arising from nuisance tripping will need to be considered. 

Therefore a 5 amp fuse or a 6 amp MCB would protect this circuit if ordinary GLS lamps were being used. However, if extra-low voltage or discharge lighting is connected to this circuit and protected by a type B MCB we must take account of the inrush current which occurs at switch-on and sometimes causes unwanted or nuisance tripping of the MCB. To avoid this, the circuit will be adequately protected by a 10 amp MCB. 

Generally fail safe Failure modes and failure rates can be quantified. Prone to nuisance trips. Trip amps are a weakness. 

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