Temperature Monitoring & Protection

“The temperature-rise of a current carrying part is the difference between the temperature of that part and the ambient air temperature.”

Temperature Rise = Temperature of the part – Ambient Air Temperature

Depending on the design of various current carrying parts, temperature rise of various parts will defer but the final temperature rise shall be within the limits as per IEC 60947-2 such that it shall not cause any kind of damage to the insulating parts or the equipment itself.

The following table gives the permissible temperature-rise limits as per IEC 60947-2 for Breaker terminals & accessible parts:

From the above table, for a circuit breaker in free-air, the accepted temperature-rise on the terminals is ΔT=80, therefore taking as reference an ambient temperature T_A=40°C, it can be deduced that the maximum permissible temperature at terminals is T_T = ΔT + T_A = 120°C.

In cases where ambient temperature is different, the temperature-rise limit varies likewise as per the above equation.

• If ambient temperature T_A =50°C, the accepted temperature-rise limit (ΔT) shall be 70°C
• At T_A =35°C, the accepted temperature-rise limit (ΔT) shall be 85°C

In circuit breakers, the TERMINALS are the critical part where overheating is prominent since there is a joint between the bus bar and the breaker terminals.

Following are some of the major causes of overheating at terminals:

1. Overcurrent

ACB is terminated with external busbars by using links. Joint resistance between ACB terminals & external links plays vital role as far as failure of the ACB due to temperature rise is concerned.

If the connected load draws over current, then the Breaker terminals and Busbar overheat. The resistance of the Busbar conductors & external links increases with temperature and contributes to further heating.

This is a cumulative effect and therefore will lead to rapid temperature rise.

2. Improper termination (loose or over tight)

If the bus bar joints are not tightened with recommended torque, it will result in temperature rise at breaker terminals.

Some of the reasons for improper termination are:

• Complicated linkwork

For example inadequate support to the linkwork and introducing additional joints would complicate the linkwork and would add extra stresses to the adjoining terminals which will then cause overheating.

In above image, an additional joint has been introduced in the linkwork at the bottom side for R and Y phases due to C‐bending. This linkwork is not supported. Hence the entire weight of the same is borne by the terminals.

• Improper alignment/absence of gap between the flats of the links/busbars

This reduces the overall heat dissipation capacity of the system.

Staggered stacking is advisable over flat stacking so that there should be sufficient gap between the flats of busbars for effective heat dissipation

• Use of improper hardware

For example use of insufficient no. of bolts and/or improper sized hardware would lead to an increase in the contact resistance which will further increase heat generation at these points.

• Over tightening of joints

Over‐tightening of joints lead to excessive stress on links/ terminals & leads to overheating. It is also responsible for washout of threads of bolt & nut.

3. Insufficient ventilation & Volume of enclosure:

In cases where Breaker is mounted inside a completely enclosed cubical, limitation is imposed on the heat generated by the breaker from being effectively carried away, which in turn increases the overall temperature. This problem can be tackled by optimizing the ventilation inside the enclosed panel by using good termination practices.

4. Harmonics Contaminated Environment:

In harmonic rich environment (e.g. IT Industries), effective current is increased due to higher order harmonics & thereby increasing overall copper losses & consequently the overheating.

5. Corrosive Atmosphere:

This problem usually arises in fertilizer plants, mills, sugar industries, refineries, paper and chemical factories where presence of corrosive gases affect the conducting materials used in Switchgear. These corrosive materials form compounds with the Silver plated film typically used on the terminals of ACBs. These compounds have much higher resistivity, which causes overheating during usage.

The effects of overheating at breaker terminals are detrimental to the circuit breaker as well as other equipments inside the panel.

Some major effects are given below:

• Reduction in the life of Bus-bar/cables & even Breakers
• Insulation deterioration and eventually insulation failure
• Deterioration of Busbar insulating sleeve as overheating results in melting of Bus bar insulating sleeves
• Blackening or Tarnishing of silver plated copper components
• Overheating can also lead to uneven expansion of the busbar and could result in undesirable operation
• Increased power losses and low reliability
• Temperature rise at terminals may eventually result in fire hazards

Thus, overheating is one of the major concerns in ACBs (or Switchgear) and is therefore needed to be controlled at every cost.

• Generation of heat at Main contacts of ACB is inevitable & depends upon the design efficiency and the quality of materials used. Thus, as a user this is not in our hands to control.
• Overheating at terminals is the bigger concern. It can be reduced by using correct termination practices & ensuring proper Ventilation inside the panel. The temperature rise can be kept in check by monitoring the temperature at the terminals.

Therefore temperature at the terminals shall be monitored continuously to check for overheating that can have adverse impact on the system & subsequently corrective measure shall be taken in order to protect the equipment & the system from any damage or interruptions.

From the above discussions we understand that it is imperative to monitor the temperature at the Breaker Terminals and provide protection against abnormal Temperature rise. This is precisely what a Temperature Module does. It is provided exclusively with OMEGA Range of Air Circuit Breakers as an accessory. It is referred as TM henceforth in the note.

• The TM monitors the temperature at the breaker terminals continuously and provides protection by tripping the breaker and/or by giving an alarm
• Located inside the breaker, it senses real time temperature through thermistors placed on the breaker terminals and provides information on the Release display
• 24V dc Spike free power supply is required for its functioning
• The TM is given as a supplementary module and works with Matrix Releases UW-MTX3.5 and above

TEMPERATURE (TM) MODULE

• Temperature protection range is 85-115°C
• Communicates with MTX Release on CAN bus (Controlled Area Network)
• Factory fitted option
• Provision for self-diagnostic test

Note: As TM is a factory fitted option, the connections are internal to the breaker and depiction here is purely for understanding purpose

• The Temperature at the terminals can be seen on the display of Matrix Release.
• The provision is made to trip the Breaker and/or give an alarm when the temperature at Breaker terminals goes beyond the specified limit which is settable pick-up range from 85°C to 115°C (configured through MTX Release).
• It takes temperature inputs through thermistors from all the four terminals (NRYB) of air circuit breaker.
• The connections of the TM (AS+, AS-, CAN-H, CAN-L) is brought to SIC from where it is connected to PS Module & Release.

In the MTX Release, follow the given sequence to configure the settings of the Temperature Module

The setting options under Module settings & Temperature as seen on Release is shown below

Let’s understand the functions and working of each of these parameters under temperature one by one:

1) Module 1: Enable for Temperature Rise Monitoring & Protection.

2) Module 2: Disable for Fr-1/2 ACBs & Enable for Fr-3 ACBs as 2 TM’s are mounted inside Fr-3 Breakers.

3) Protection: Enable if protection and metering are required, Disable if only metering is required.

4) Mode: Three options can be selected as shown below:

Alarm if only alarm is required when temp. at the terminals crosses the Pick-up value.

Trip if ACB needs to be only tripped when temp. crosses the Pick-up value.

Both if alarm is needed & ACB should be tripped.

5) Prealm: 0.5Tp to 0.95Tp in the steps of 0.05Tp (where Tp is Temperature Pick-up value)

Starts giving pre-alarm as soon as the temp. exceeds the Prealm value.

6) Pickup (Tp): 85°C to 115°C in step of 0.1°C

Pick-up value is the temp. at which the TM should start giving protection (Trip/Alarm/Both).

7) Delay: 0 min to 15 min in step of 1 min

Delays the tripping of the breaker by the set delay time.

Therefore through the use of TM, complete monitoring of Temperature and protection against the same at Breaker terminals is achieved. Real time temperature at the terminals is brought on the Release display and complete Protection in terms of tripping the ACB, Alarm & Pre-alarm features makes it the most reliable Temperature Monitoring & Protection system.