Emergency Stop Switch Spurious Trip

Today we will discuss about one of the most important components used in the Instrumentation system i.e. NC contact. This is widely used in Emergency Stop or Stop pushbutton as a fail-safe condition is needed there.

This system may lead to spurious trips in long run. Yes, you read it right SPURIOUS TRIP!

Internal Components of Switch

The switch Normally-Close (NC) contact used has a simple internal as shown below. The main components are spring and contact in a switch.

Now as per the fail-safe concept, the stop button contact should be closed in non-actuated conditions for many hours and days as per the required application. Whenever the pushbutton is pressed, which is maintained mainly by a spring and contact, the contact should open.

Now due to the aging effect, this spring may fail or its strength may be lost after some time. Sometimes these issues may create an open contact and actuate the pushbutton without actually pressing the stop button, which is a SPURIOUS TRIP.

Problem Solution

To avoid these types of problems, 2 out of 2 configurations should be used. For this, 2 sets of contacts in a single pushbutton must be used in place of 1 set of contact of the Normally-closed push button.

Whenever both contacts are actuated then only, stop/trip action should take place i.e. both contacts should be connected in parallel.

If the switch contacts are taken as Digital Input in PLC or DCS logic, then 2 different DI should be used so that the health of both pushbuttons can be monitored in the logic, i.e. alarm should appear whenever a contact is actuated such that respective action can be configured in the logic.

How to Check Switch Contact Status?

In non-actuated conditions (for NC contact), resistance should be below 1 ohm, while in the actuated condition it should give resistance in mega Ohm.

When to Check Switch Contact healthiness?

We have to check the switch contact healthiness every 3 years or during a major plant shutdown.

-END-

Comments

Ferrules and Cross Ferruling

Ferrules are identification labels provided for every wire terminations in an instrument, equipment, or electrical/instrumentation control panels. These tube-shaped sleeves can be inserted easily on each individual wire in a multi-core cable. In earlier days fixed digits/letters are used as ferrules, but now Instrumentation engineers/technicians prints out desired ferrules by using a ferrule printing machine. Typical Ferrule The numbers/ letters on the ferrules will be given as per the approved electrical hook up or loop diagrams. This helps technicians to easily identify a particular loop/wiring from a series of terminal blocks and to troubleshoot the desired terminal connection. Separate numbers on the ferrules distinguish the positive and negative polarities of wires, thus ensure the polarity protection of the instrument. Cross Ferruling As a wire is connected on its both ends, it is quite useful to use a cross reference method for wire identification. Unlike normal ferru...

What is a Torbar? – Averaging Pitot Tubes

The Torbar is employed for flow measurement of liquids, gases, or steam in circular, square, or rectangular section ducts for large flow rates. The Torbar is an insertion type multi-port self-averaging primary sensor for flow measurement. Torbar TORBAR is a set of Pitot tubes mounted on a bar across the pipeline with no moving parts. An averaging Pitot tube is a technology, while TORBAR is a manufacturing brand name. There are several brands available in the market with VERABAR, ANNUBAR, etc. Averaging Pitot Tube Principle Purpose Averaging Pitot tube can be employed when the average velocity of the flow profile, rather than the velocity in a specific point in the cross-section is desired. Averaging Pitot Tubes Principle It measures the differential pressure between the static pressure tap and the tap of full pressure of a stream. Thus such magnitude of differential pressure is directly proportional to the square of the flow rate. Working The TORBAR is designed in such a way that ...

Dissolved Oxygen Analyzer Working Principle

What is dissolved oxygen ? Dissolved oxygen refers to oxygen dissolved in water. Its concentration is expressed as the amount of oxygen per unit volume and the unit is mg/L. Biologically, oxygen is an essential element for respiration of underwater life and also acts as a chemical oxidizer. The solubility of oxygen in water is affected by water temperature, salinity, barometric pressure, etc. and decreases as water temperature rises. Measurement of dissolved oxygen by the membrane electrode method The membrane electrode method measures a diffusion current or reduction current generated by the concentration of dissolved oxygen or partial pressure of oxygen to obtain the concentration of dissolved oxygen. This method is not affected by the pH value of water being measured, oxidation and reduction substances, color, turbidity, etc. and the measurement method offers good reproducibility. When a sensor is inserted into water, an air layer forms on the membrane (Teflon membrane). The ox...

TILI Automation

Search This Blog