Why does a loose connection cause heat?

Hardly any other measurement method provides such quick information on the status analysis of electrical switchgear at all voltage levels. If these checks are not carried out in a timely and regular manner, you will usually be punished for these failures in retrospect.

VdS certificate for recognition as an expert for electrothermography

Certificate no .: ET 06008
valid until June 21st, 2022 (VdS Schadenverhütung GmbH, certification body, 50735 Cologne)

Certificate as a pdf file
(151 KB)

Thermovision on electrical switchgear, also known as thermography / thermography, has been carried out by energy supply companies for many years. Regular controls using infrared technology ensure that the electrical systems and thus the power supply are highly available. These preventive maintenance measures are also carried out in a large number of companies. Regular infrared measurements result in economic benefits, which fire insurance companies also reward by reducing insurance premiums. In order to minimize the dangers and to rule out incorrect measurements and misinterpretations by the measuring staff as far as possible, the insurance industry has insisted that the VdS Loss prevention a certification of electrical specialists according to DIN VDE 1000-10 and electrical engineers for "VdS recognized expert for electrothermography VdS 2859: 2005-01". These certifications have been offered by the VdS since 2005 and are awarded after a one-week training course and passing the exam. This certificate has been required by the insurance industry for a number of years.

Purpose and benefits of thermography:

  • Documentation of system conditions and potential risks
  • Early detection of weak points and damage
  • Increase in system availability and reliability
  • Avoidance of consequential damage
  • Reduction of fire and accident risks

Through the VdS (Gesamtverband der Deutschen Versicherungswirtschaft e.V., Damage Prevention Office) is the inspection of electrical systems by VdS 2858 described in more detail.
According to this, electrical systems must be checked regularly by the operator (recurring tests), e.g. in accordance with:

  • Technical test regulations of the respective federal state,
  • BGV A3 (accident prevention regulation (UVV) of the employer's liability insurance association),
  • DIN VDE 0105, in which the "correct condition of the electrical system" is to be determined,
  • Fire insurance clause (Clause 3602), which additionally requires an examination according to the safety regulations of fire insurance companies.

Thermography cannot replace the recurring tests mentioned above. Nor is it a substitute for the necessary visual inspections, functional tests, current measurements, etc., which must be carried out as part of the aforementioned periodic tests. However, it is a helpful, supplementary measurement method and enables, in particular, examinations and evaluations of the system status that were previously only possible with great difficulty or with great effort. A great advantage is that the measurements can be carried out while the system is running, i.e. when it is live. Today, thermography is part of the state of the art in security technology.

. . . . . . . .

Around 35% of all company fires are caused by thermal heating of electrical systems. After the second thermographic examination, the failure rate drops by 80%.
The left photo shows the compensation of a 0.4 kV system which burned out, because the contact resistance at the screw connection on conductor L1 (within the red circle) was so great due to a loose connection that the connection point began to glow and the cabinet on fire sat. Fortunately, in this case the fire was limited to the closet, as it was far enough away from other flammable objects. After this fire, the industrial company in question decided, at the insistence of the insurance company, to take annual infrared recordings for preventive fire protection.


Source: www.feuerwehr-erfurt.de/

The two pictures below show a 110/10 kV transformer. In the left infrared image (taken 20 years ago with what is now outdated camera technology) a temperature of 80.9 ° C can be measured on the left transformer bushing. At first it was suspected that the screw connection on the transformer bushing was faulty. More detailed investigations, such as the oil gas analysis, showed that the connection within the transformer was faulty and had to be replaced.
The highest temperature we measured on 110kV systems was 530 ° C with a load of only 27%. The extrapolation showed that at nominal load (bottleneck current) the affected screw connection of the 110kV line would have a temperature of approx. 3,000 ° C. However, this high temperature would never be reached, as the aluminum melts at approx. 660 ° C and thus interrupts the power supply. If many industrial companies are affected, the costs, especially due to the power failure, can become very high for the energy supply companies.

 

 

. . . . . . . .

ladder

 

:

L1

L2

L3

 

Nominal load

(A)

:

1.375

1.375

1.375

 

Load during the measurement

(A)

:

920

920

920

 

Load in percent

(%)

:

67

67

67

 

maximum object temperature

(° C)

:

43

43

81

 

measured overtemperature

(D. T in K)

:

-

-

38

 

Temperature at nominal load

(° C) approx.

:

-

-

160

 

Error group

(1 - 4)

:

-

-

3

 

In evaluating the upper infrared image of the transformer bushing, the table gives an overview of the load on the individual conductors during the measurement. With a load of 67% of the nominal load, the temperature at the top of the candle was 81 ° C. If you extrapolate this temperature to the nominal load (bottleneck current), the result would be a temperature of approx. + 160 ° C in the upper part of the transformer bushing. At the direct point of failure, of course, significantly higher.
In general, it is very difficult to estimate errors that are not directly visible. Due to the pure excess temperature, there is an error group classification into error group 3, although the classification into group 4 would have been more correct.
After opening the transformer, the major damage could be seen according to the adjacent photo. It is very easy to see that the first expansion band has already burned away and it is hardly possible to speak of a screw connection. It would not have taken long and the expansion tape for the passage inside the transformer would have burned out completely due to the high temperatures. In this case, the power supply to a larger part of the city would have been interrupted. The repair costs amounted to 85,000 EUR. In the event of an unexpected, lengthy power outage, the costs would have been much higher.

. . . . . . . .

Warming of a fault location by more than double within a year

Since the lower point of failure could not be eliminated immediately for operational reasons, the point of failure was of course localized again one year later, after another regular check of the entire electrical system. After a year, the temperature has more than doubled if the load remained the same. These two infrared images are also of an older date and their resolution cannot be compared with the current infrared images.
The table below shows the extrapolation to the nominal load for this thermal error. The two infrared images below were taken 20 years ago using a technology that is no longer up-to-date. However, they are interesting examples that are rarely available. However, this was "state of the art" 20 years ago.

maximum temperature 68 ° C on March 22nd, 1996

. . .

maximum temperature 136 ° C on 02.26.1997

. . . . . . . .

ladder

 

:

L1

L2

L3

Nominal load

(A)

:

1.000

1.000

1.000

Load during the measurement

(A)

:

500

500

500

Load in percent

(%)

:

50

50

50

maximum object temperature

(° C)

:

136

35

35

measured overtemperature

(D. T in K)

:

101

-

-

Temperature at nominal load

(° C) approx.

:

400

-

-

Error group

(1 - 4)

:

4

-

-

. . . . . . . .

Original photo of the weak point shown in the infrared image from 1997. No warming or discoloration can be seen on the aluminum, the copper or the screws.

You should avoid connecting copper and aluminum directly with one another, since problems are always to be expected due to the galvanic element that is created in this way (are very far apart in the voltage series in the periodic system). If moisture is also expected, problems are inevitable. If such a connection cannot be avoided, so-called AlCu washers can be used. These have a copper surface on one side and an aluminum surface on the other.

Often it is only small things that cause machine breakdowns or, in the worst case, fires. The lower photo with the corresponding infrared image show loose cables on a 0.4kV main switch. No discoloration of the insulation is noticeable to the eye. A sign that the fault has not been around for a long time. Often there are loose terminal points that cause the contact resistance to rise and thus lead to heating. Tightening the connection points usually fixes the error, with this mostly copper cable and the low temperatures. If the temperature is higher, the connection cables must be replaced or replaced. An exchange, such as the switch in this example, may then be necessary.

. . . . . . . .

At an ambient temperature of only 13 ° C, a temperature of almost 400 ° C should be measured on the lower screw-locking element of the light distribution. The high temperature of the fuse cannot be seen from the outside. Perhaps the screw lock was not tightened enough. These screw fuses tend to get hot very quickly if the contact pressure is not high enough. Sometimes the cable connection on the fuse base is also responsible for the heating. An exchange of the entire fuse element with the bracket is necessary here. The cable connection must also be checked and, if necessary, the cable must be replaced or readjusted.

. . . . . . . .

On this 0.4kV NH fuse (conductor L2), 403 ° C was measured on the upper contact tongue. Inadequate spring contact will have been the cause of this strong warming. A strong discoloration of the contacts can already be seen in the original image. The protective plastic hose over the fuse is already beginning to dissolve. Immediate action must be taken here to prevent a fire and counteract production downtimes. An exchange of fuse holder, fuse and also the supply cable is necessary.

. . . . . . . .

The rail connection under the cladding is not in order on this BD system. 48.2 ° C is not very warm yet, but there is clearly a weak point. Since a warming will certainly increase in the long run (see the warming of a fault point by twice as much within a year - on this page) there is also a need for action here.

. . . . . . . .

The feed line to a compressor at the outlet of this circuit breaker (conductor L2) is so hot at 423 ° C that a fire could not have been avoided for long. The outlet had only been put into operation a year ago. A burr created when drilling open the cable lug was not removed in this example. This prevented the contact surfaces from resting evenly on top of one another. The resulting cross-sectional weakening caused the strong warming. An almost 100% duty cycle with a constant load of approx. 210A made the screw of the actual connection glow. This screw head cannot be seen directly because it is on the opposite side of the cable lug. The glowing screw head was already reflected in the galvanized sheet metal of the switch cabinet, already visible to the naked eye. Due to the high temperature at the connection of the circuit breaker, it is advisable to replace the entire circuit breaker.

. . . . . . . .

A defective area can be seen on the inside of this 110kV circuit breaker in the area of ​​the upper arcing chamber.
The gray representation could mean that it is a photo and not an infrared image. Such photo-realistic radiometric infrared images with all temperature information are only possible with high-resolution infrared systems and are inconceivable with LowCoast devices.

. . . . . . . .

The spring for the contact pressure of the entry contacts of this 110kV pantograph busbar isolator is broken. Temperatures of 235 ° C are measured on the impact side, directly at the contact. On the back it is even 336 ° C. The burned color can already be seen here. With a load of 67% of the nominal load, extrapolation results in a temperature of approximately 700 ° C. This separator was immediately taken out of service and parts replaced ...

. . . . . . . .

 

 

 

. . . . . . . .

With this 380kV surge arrester, only the voltage is present. As a rule, there is no current flow here. Nevertheless, the head on conductor L3 heats up. The enlarged view with the 7 ° infrared telephoto lens confirms this warming again. Although the temperature increase is only 4 ° C, it is reliably localized using suitable infrared technology. Such defects cannot be found with cheap infrared cameras.

. . . . . . . .

 

 

 

. . . . . . . .

The lower infrared image shows a flat connection on a 380kV circuit breaker connection. Due to the great distance, this detailed image would not be possible without a telephoto lens and an exact assignment of the weak point could not be made. At the time of the measurement, the thermal weak point had a temperature of 230 ° C with only 14% of the nominal load. If this temperature is extrapolated to the limit current, the theoretical temperature would be around 2,600 ° C. However, this high temperature would never be reached because the aluminum of the flat connection would melt at approx. 600 ° C (melting point depends on the alloy). Here, too, the use of unsuitable measurement technology would have resulted in incorrect measurements. With a camera that cannot record high temperatures, the temperature could not even be measured and without a suitable telephoto lens or only with a small screen in strong sunlight (as with the inexpensive infrared cameras) one would not have recognized the error. This could have led to serious damage and large-scale power outages. With all measurements in the substations, but also in industrial companies, a very high level of responsibility rests on the measurement engineer and the commissioning energy supply company or the technical managers in the industrial companies must always be able to fully rely on the measurements. For this reason, camera technology appropriate to the intended use is just as important as adequate qualification and certification of the measurement engineers commissioned with the measurement tasks. The test engineer for checking electrical systems should have professional experience and must be a qualified electrician in accordance with DIN VDE 0105-100. He should have the certification for the examination of electrical systems level 2 according to DIN EN 473, since only this authorizes independent work without instruction. Furthermore, proof of the VdS-recognized expert for electrothermography for the examination of electrical systems is required and is required by insurance companies. Only these prerequisites lead to a high level of expertise in the measurement and assessment of thermal warming, which is often also component-related and normal.
Opening the contact point with subsequent cleaning and greasing of the contact surfaces and renewed screwing using a torque wrench usually eliminates the weak point. With these flat connections, simply tightening the screws is only successful in the rarest of cases. Here it is necessary to investigate further whether the high temperatures have not already caused damage to the parts.

. . . . . . . .

Insufficient contact pressure on this 380kV scissor separator resulted in a temperature rise of over 104 ° C in the example below. Due to the large measuring distance of 380kV systems, detailed recordings are only possible with a 7 ° infrared telephoto lens. In addition to insufficient contact pressure, the cause of the heating can also be the surface properties of the contact points. Burn-in points or oxide layers can increase the contact resistance so much that the components heat up. The measurement in the atmosphere results in a low temperature of over -60 ° C (cloudless sky) in this infrared image.

. . . . . . . .

. . . . . . . .

As already mentioned, it is imperative to use high-quality infrared technology with telephoto lenses to check high-voltage systems. This is explained by the geometric resolution (IFOV - I.nstantaneousF.ield OfView) of each IR camera. At a measuring distance of 10 m, this geometric resolution, e.g. the ThermaCAM PM 695 with a 24 ° lens, is 13 mm. It is therefore only possible to measure the exact temperature of an error at a distance of 10 m, which does not fall below an expansion of 13 mm. If you now use a 7 ° telephoto lens, this geometric resolution improves to 3.8 mm. With a telephoto lens, you can localize significantly smaller flaws and determine their temperature exactly. The differences between the various lenses can be seen in the two lower, older infrared images 026 and 033.

The two infrared images below are also intended to illustrate the need for a telephoto lens. This is a 110kV T-terminal at a height of approx. 8m. With the 7 ° telephoto lens, you can see very precisely which screw connection or spring plate on the terminal is faulty. The further infrared image was saved with a 24 ° normal lens. Here you can only see that the T-terminal is faulty. The reason for the warming is not possible due to the large measuring distance.

Since telephoto lenses are very expensive, only a few similar offices have these lenses available. Especially with 110kV, 220kV and 380kV systems the viewing distances are very large and telephoto lenses are according to our experience

The lower left infrared image of a 110kV T-clamp was taken with a 24 ° normal lens recorded. 36 ° C are measured here.

. . . . . . . .

The right infrared image of the same T-clamp was made with a 7 ° telephoto lens saved. The IR image shows a temperature of 61.4 ° C.

If unsuitable lenses are used, serious incorrect measurements can occur, as shown by the two infrared images below. At this 110kV T-terminal of an overvoltage connection, a telephoto lens can be used to measure a very small but significant excess temperature of 320 ° C. With a normal lens, this flaw can also be recognized in accordance with the left infrared image. The temperature is displayed too low by 124 ° C due to the large measuring distance and the small size of the overtemperature range. The reason for this is to be found in the geometrical resolution (IFOV) of the infrared cameras mentioned at the beginning. So you can't measure small objects at great distances with unsuitable lenses. You can find more such examples in the newspaper article "Optimally examine high-voltage systems using infrared thermography" (430 KB).

Infrared recording of a T-clamp from a surge connection with a standard lens. The highest measured temperature is with + 196 ° C measured.

. . . . . . . .

Infrared image of a T-clamp with a telephoto lens. The highest measured temperature is with + 320 ° C measured even though the two IR images shown show the same terminal.

 

 

 

. . . . . . . .

In order to enable a classification of the fault locations, a fault group classification was developed for better quantification.
The excess temperatures Delta (D) T are thus divided into the following 4 error groups:

Classification of error groups for measured overtemperatureD.T inK

Temperature classification

0 K <D.T <10 K

10 K <