MTTR and MTBF, what are they and what are their differences?

MTBF (Mean Time Between Failures) and MTTR (Mean Time To Repair) are two very important indicators when it comes to availability of an application. Despite its importance in the performance of the processes, most managers do not make full use of these key performance indicators (KPIs) in their control activities. Find out in the next few lines the differences between these two metrics and how they can be used to improve the efficiency of the processes in your company.

What are MTBF and MTTR:

MTBF, or Mean Time Between Failures, is a metric that concerns the average time elapsed between a failure and the next time it occurs. These lapses of time can be calculated by using a formula.

Whereas the MTTR, or Mean Time To Repair, is the time it takes to run a repair after the occurrence of the failure. That is, it is the time spent during the intervention in a given process.

The two formulas Conceptual differences, different formulas! Check the ways to calculate MTBF and MTTR:

MTBF:

Total time of correct operation in a period/number of failures

For example: a system should operate correctly for 9 hours During this period, 4 failures occurred. Adding to all failures, we have 60 minutes (1 hour). Calculating the MTBF, we would have:

                                        MTBF = (9-1)/4 = 2 hours

This index reveals that a failure in the system occurs every 2 hours, leaving it unavailable and generating losses to the company.The opportunity to spot this index allows you to plan strategies to reduce this time.

MTTR:

Total hours of downtime caused by system failures/number of failures
Using the same example, we come to the MTTR, by using the following formula:

                                     MTTR = 60 min/4 failures = 15 minutes

Above, we have the average time of each downtime. Therefore, the company knows that every 2 hours, the system will be unavailable for 15 minutes. Being aware of our limitations is the first step to eliminate them.

Uptime calculation:

The uptime calculation involves MTTR and MTBF. We can get to the uptime of a system, for instance, using these 2 KPIs. 

Let’scheck the formula:

                                      Uptime= MTBF/(MTBF + MTTR)


To be more clear, nothing better than a practical example. Imagine the following situation:
A. How long the system should work: 36 hours
B. How long the system was not working: 24 hours
C. How long the system has been available: 12 hours
D. A total of 4 failures occurred.

          Uptime: (A-B/D) / [(A-B/D) + (B/D)] = (36-24/4) / [(36-24/4) + (24/4)] = 3 / 9 = 33%

Benefits in the use of these performance indicators:

1. MTTR and MTBF are two indicators used for more than 60 years as points of reference for decision-making. MTBF is a basic measure of the reliability of a system, while MTTR indicates efficiency on corrective action of a process.
2. If the MTBF has increased after a preventive maintenance process, this indicates a clear improvement in the quality of your processes and, probably, in your final product, which will bring greater credibility to your brand and trust in your products. The MTBF increase will show that your maintenance or verification methods are being well run, a true guide to support teams.
3. In the case of MTTR, the effort should be exactly the opposite: to reduce it as much as possible to avoid loss of productivity for system unavailability. A lower mean-time-to-repair indicates that your company has quick answers to problems in their processes, which demonstrates a high degree of efficiency.
4. As it can be noticed, MTTR and MTBF are two powerful performance indicators that should be used to expand the company’s knowledge about processes and reduce losses in productivity or quality in the products offered.

Why is earthing necessary in electrical systems?

Earthing is nothing but a return path of fault/excessive currents safely into the ground in the presence of minimal resistance or impedance.

1. The primary purpose of earthing is to reduce the risk of serious electric shock from current leaking into uninsulated metal parts of an appliance, power tool, or other electrical devices. 
2. In a properly earthed system, such leaking/fault current is carried away harmlessly while tripping the fuse.
3.  Earthing also provides protection from large electrical disturbances like lightning strikes and power surges.

What is the difference between earthing, grounding and neutral?

Earthing:

Earthing means connecting the dead part, which does not carries current under normal condition to Earth.

The purpose of Earthing is to minimize risk of receiving an electric shock if touching metal parts when a fault is present. 

Generally GREEN wire is used for this as a nomenclature.

Grounding:

Grounding means connecting the live part  which carries current under normal condition to the earth for example neutral of power transformer. It is an an effective return path from the machine to the power source.

Grounding is nothing but neutral point of a star connected transformer.

Generally BLACK wire is used for this as a nomenclature.

Difference between earthing & Grounding:

There is no major difference between earthing and Grounding, both means “Connecting an electrical circuit or device to the Earth”. This serves various purposes like to drain away unwanted currents, to provide a reference voltage for circuits needing one, to lead lightning away from delicate equipment.

(1) Difference in Terminology:

In USA term Grounding is used but in UK term Earthing is used.

(2) Balancing the Load Vs Safety: 

Grounding is done to balance unbalanced load. While earthing is used between the equipment and earth pit so as to avoid electrical shock and equipment damage.

(3) Equipment Protection Vs Human Safety:

Earthing is to protect the circuit elements whenever high voltage is passed by thunders or by any other sources while Grounding is the common point in the circuit to maintain the voltage levels.

(4) System Zero Potential Vs Circuit Zero Potential:

Earthing and Grounding both is refer to zero potential but the system connected to zero potential is differ than Equipment connected to zero potential .If a neutral point of a generator or transformer is connected to zero potential then it is known as grounding. At the same time if the body of the transformer or generator is connected to zero potential then it is known as earthing.

Corona Discharge

In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized. The discharge will occur when the strength (potential gradient) of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects.

 

Problems caused by corona discharge

Corona discharge is generally undesirable. It represents 

1. Power loss
2. Audible noise
3. Electromagnetic interference
4. Purple glow
5. Ozone production
6. Insulation damage

Also Coronas can generate audible and radio-frequency noise.

Mechanism of corona discharge

Corona discharge results when the electric field is strong enough to create a chain reaction: electrons in the air collide with atoms hard enough to ionize them, creating more electrons which ionize more atoms. The process is: 

1.   A neutral atom or molecule, in a region of strong electric field (such as the high potential gradient near the curved electrode) is ionized by a natural environmental event (for example, being struck by an ultraviolet photon or cosmic ray particle), to create a positive ion and a free electron.

2.   The electric field accelerates these oppositely charged particles in opposite directions, separating them, and preventing their recombination, and imparting to each of them kinetic energy.

3.   The electron has a much higher charge/mass ratio and so is accelerated to a higher velocity than the ion. It gains enough energy from the field that when it strikes another atom it ionizes it, knocking out another electron, and creating another positive ion. These electrons are accelerated and collide with other atoms, creating further electron/positive-ion pairs, and these electrons collide with more atoms, in a chain reaction process called an electron avalanche. Both positive and negative coronas rely on electron avalanches. In a positive corona all the electrons are attracted inward toward the nearby positive electrode and the ions are repelled outwards. In a negative corona the ions are attracted inward and the electrons are repelled outwards.  

 

4.   The glow of the corona is caused by electrons recombining with positive ions to form neutral atoms. When the electron falls back to its original energy level, it releases a photon of light. The photons serve to ionize other atoms, maintaining the creation of electron avalanches.

 

 5.   At a certain distance from the electrode, the electric field becomes low enough that it no longer imparts enough energy to the electrons to ionize atoms when they collide. This is the outer edge of the corona. Outside this the ions move through the air without creating new ions. The outward moving ions are attracted to the opposite electrode and eventually reach it and combine with electrons from the electrode to become neutral atoms again, completing the circuit.

 

Thermodynamically, a corona is a very non equilibrium process, creating a non-thermal plasma. The avalanche mechanism does not release enough energy to heat the gas in the corona region generally and ionize it, as occurs in an electric arc or spark. Only a small number of gas molecules take part in the electron avalanches and are ionized, having energies close to the ionization energy of 1 - 3 ev, the rest of the surrounding gas is close to ambient temperature.The onset voltage of corona or corona inception voltage (CIV) can be found with Peek's law (1929), formulated from empirical observations. Later papers derived more accurate formulas.