Construction/operation of electromechanical IDMTL relay [Industrial Electrical Power Systems]

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As the name implies, it’s a relay monitoring the current, and has inverse characteristics with respect to the currents being monitored. This (electromechanical) relay is without doubt one of the most popular relays used on medium- and low-voltage systems for many years, and modern digital relays' characteristics are still mainly based on the torque characteristic of this type of relay. Hence, it’s worthwhile studying the operation of this relay in detail to understand the characteristics adopted in the digital relays.

Typical mechanical relay

The above relay can be schematically represented.

From CTs A I2 I1 Disk; B Braking magnet

The IDMTL relay

The current I1 from the line CTs, sets up a magnetic flux A and also induces a current I2 in the secondary winding which in turn sets up a flux in B. Fluxes A and B are out of phase thus producing a torque in the disk causing it to rotate. Now, speed is proportional to braking torque, and is proportional to driving torque. Therefore, speed is proportional to I ².

But, Distance Speed Time

= Hence, 2 Distance 1; Speed; Time I

== This therefore gives an inverse characteristic. Definite minimum time t I

Characteristic curve of relay

It can be seen that the operating time of an IDMTL relay is inversely proportional to a function of current, i.e. it has a long operating time at low multiples of setting current and a relatively short operating time at high multiples of setting current. The characteristic curve is defined by standards and is shown. Two adjustments are possible on the relay, namely:

1. The current pick-up or plug setting: This adjusts the setting current by means of a plug bridge, which varies the effective turns on the upper electromagnet.

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2. The time multiplier setting: This adjusts the operating time at a given multiple of setting current, by altering by means of the torsion head, the distance that the disk has to travel before contact is made.

Current (plug) pick-up setting

This setting determines the level of current at which the relays will pick-up or its disk will start to rotate. IEC142 states that, the relay must definitely operate at 130% setting and reset at 70% setting. In this context, the plug setting is that current at which the operating and restraining torques are in a state of balance. In practice, IEC142 requires that the relay should definitely not operate at the setting, and to ensure this, a relay may display a slight tendency to reset at the normal setting.

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The relay therefore normally picks up in the range of 105-130% its current plug setting.

++++ Time and pick-up errors at unity time multiplier

Usually the following ranges of nominal current are used, giving a 1:4 ratio in seven steps:

Percentage plug settings (reyrolle)

Overcurrent (%): 50% 75% 100% 125% 150% 175% 200% Ground fault (%): 20% 30% 40% 50% 60% 70% 80% Or (%): 10% 15% 20% 25% 30% 35% 40%

Current plug settings (GEC) - for 5 A relay

Overcurrent (A): 1.5 3.5 5.0 6.25 7.5 8.75 10.0 Ground fault (A): 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Or (A): 0.5 0.75 1.0 1.25 1.5 1.75 2.0

Normally, the highest current tap is automatically selected when the plug is removed, so that adjustments can be made on load without open-circuiting the current transformer.

Time multiplier setting

This dial rotates the disk and its accompanying moving contact closer to the fixed contact, thereby reducing the amount of distance to be traveled by the moving contact, hence speeding up the tripping time of the relay.

This has the effect of moving the inverse curve down the axis.

Current (multiples of plug setting)

Time multiplier setting

++++ Time/current characteristic

The above curve is the most common type used, namely the Normal inverse curve. Its characteristic shows an operating time of 3 s at 10 times the current plug setting i.e. with the plug bridge set at 1 A, when 10A flows through, the relay will close its contacts after 3sec. (i.e. with the time multiplier set at 1.0). The time of operation of the relay is chosen by collectively selecting the current and time plug settings. There is another popular version, which has an operating time of 1.3 s at 10 times the current setting.

It’s possible to manufacture relays with different characteristics, but the principle of operation remains the same. Other characteristic curves popular are very and extremely inverse. The different time characteristic curves of an IDMTL relays is shown. These are represented in logarithmic graphs due to the exponential nature.


Burden is the normal continuous load imposed on the current transformers by the relay, normally expressed in VA or some times in ohms. For electromechanical relays, this is normally stated as 3 VA nominal. The modern electronic relays offer a much lower figure, which is one of their virtues.

However, for the electromechanical type, the selection of the plug setting does have an effect on the burden. As stated earlier, the operating coil is wound to give time/current curves of the same shape on each of the seven taps, which are selected on the plug bridge.

As there is a required minimum amp-turns of magnetic flux to get the relay to pick-up, the lower the current the more turns are necessary. The lower the setting therefore results in higher the burden on the CTs. Example:

Operating time (seconds)

Fault current (multiple of current setting) Extremely inverse Very inverse Normal inverse

++++ Fault current (multiple of current setting)

For 5 A relay on 200% tap, …

On 10% tap, ..

+--+ Electromechanical relays - coil impedance vs plug setting

Overcurrent reyrolle - TM 50-200% GEC-CDG Impedance (ohms)

Ground fault reyrolle - TM 20-80% GEC-CDG Impedance (ohms)

Ground fault reyrolle - TM 10-40% GEC-CDG Impedance (ohms)

The lower tap therefore, places a higher burden on the CTs and they must have adequate performance to meet such demands. This is often not the case in low ratio CTs. A mistaken impression is created that the relay is at its most sensitive setting when it’s set on its lowest tap. However, the fact is that the CTs may saturate under these conditions due to the higher burden, causing the electromechanical relay to respond more slowly, if at all it picks up. However, the modern digital relays don’t exhibit such behavior and have constant burden through out its operating range.

+--+ gives the burden values exhibited by one of the most common type of electromechanical relays used in the electrical systems and the curves that follow show the advantage of static/digital relays over electromechanical relays in this aspect.

++++ Impedance vs plug setting (electromechanical and digital) --Ohms Electromechanical relay.

Impedance varies according to plug setting Impedance of 1A relay 50-200% Electronic relay impedance constant at a very low level over whole range.


Since an electrical system employs many relays, mechanical or electrical flag indication is provided in each relay to indicate whether that relay has operated to indicate the type of fault involved. Many modern relays are of the draw out type so that, the relay can be removed from its case even when the CT circuits are alive. This is possible as the associated CT terminals in the case are short circuited just before the relay contacts break whilst the relay is being withdrawn.

Certain models also have catches, which hold the relay in its case. When these catches are unlatched, the tripping circuit is opened so that accidental closing of the trip contacts won’t trip the associated circuit breaker. This feature must not always be relied upon to prevent tripping as it does not necessarily isolate all tripping circuits and the feature is not present in all relays.

Testing of IDMTL relays

Modern relays are very reliable and in their dust proof cases, they remain clean. However, dirt and magnetic particles are the biggest cause of problems in electromechanical relays.

Hence, when this type of relay is removed for testing, it should be covered, while not being actually tested. It should preferably be kept in a spare case or a plastic bag if stored or transported.

For operational tests, a load transformer and variac can be used to supply current, while a timer will indicate when the tripping contacts close. Typical connections are shown. This method can be used to check that the relay operates, that the flag drops correctly just as the contacts are made with slow disk operation and that the contacts do make positively with good pressure.

Caution: This method is not reliable for timing or pick-up tests. A proper relay current test set is necessary for accurate tests as with this simple set up, distorted non-sinusoidal currents result because of the non-linear magnetic circuit of the relay.

220 VAC Variac Load TRFR Timer

++++ Testing of IDMTL relays

In service the relay is driven from a pure current source, namely the line CTs. The voltage that is developed to drive this current through the non-linear magnetic circuit of the relay becomes distorted, but the current remains 'pure' and faithful to the primary current. When testing from the normal 220 V supply, we have a pure voltage source.

Hence, the current now becomes distorted and non-sinusoidal, giving the relay false parameters on which to operate.

Special test sets are on the market, which are designed to inject sinusoidal currents into the relays so that accurate timing and pick-up currents can be recorded. If the relay timing is found to be outside the tolerance limits, don’t attempt to rectify this by adjusting the spiral hairspring at the top of the disk shaft, as this could upset the whole characteristic.

This spring should only be adjusted by trained relay service technicians when checking for 'disc creep' and this together with adjustments of the magnets, determine the accuracy of the timing characteristics.

++++ Typical waveforms when relay driven from CTs in service Current Voltage

++++ Typical waveforms when relay driven from plain 240 V aux. supply and table of errors

++++ A 5A ground fault relay supplied from 200/5 A current transformers I_relay = I A, plug setting = 0.5A. The following table gives the margin of errors in the test results based on the testing source.


Voltage Source vs Current Source Testing of Mechanical Relays

5A Overcurrent 5A; Ground Fault 1A; Ground Fault

25A Plug Setting 0.5A Plug Setting 0.2A Plug Setting Times Time Error Time Error Time Error Current Voltage Source Current Source

Voltage Source Current Source

×2 9.15 11.27 23% 8.99 11.6 29% 10.32 12.15 18%

×3 5.85 7.44 27% 6.04 7.31 21% 6.63 8.1 22%

×5 4.25 4.97 17% 4.15 5.07 22% 4.59 5.5 20%

×10 2.85 3.2 12% 2.79 3.21 15% 3.17 3.58 13%

×20 2.1 2.2 5% 2.01 2.21 10% 2.34 2.46 5%


Setting of an IDMT relay Example: Calculate the plug setting and time multiplier setting for an IDMTL relay on the following network so that it will trip in 2.4s. 5A IDMT 100/S 1000A

++++ Example of calculation of plug and time multiplier setting

Answer: Fault current = 1000 A

CT ratio = 100/5 A Hence expected current into relay under fault conditions, …

Choose plug setting of 5 A (100%). Therefore, current into relay as a multiple of plug setting during fault: 50 10 times 5

== We require the relay to operate after 2.4 s as soon as this much current starts flowing in the circuit. Referring to characteristic curves below, read time multiplier setting where 10 times plug setting current and 2.4 s cross, which is about 0.8. Accordingly, relay settings = current plug tap 5 A (100%) and time multiplier 0.8. Alternatively, if the current plug setting is chosen as 125% (6.25 A), the fault current through the relay will be 50/6.25 = 8 A. The graph shows that eight times plug setting to operate in 2.4 s, the time multiplier should be about 0.7. This technique is fine if the required setting falls exactly on the TM curve. However, if the desired setting falls between the curves, it’s not easy to estimate the intermediate setting accurately as the scales of the graph are log/log. The following procedure is therefore recommended. Time current characteristic Inverse time relay 50 and 60 cycles/s

12 20 330 4 5 68 7 910 Operating time (seconds); Current (multiples of plug setting); Time; Multiplier Setting

++++ Multiples of plug setting current

Go to the multiple of plug setting current and read the seconds value corresponding to the 1.0 time multiplier curve. Then divide the desired time setting by this figure. This will give the exact time multiplier setting:

Seconds value at 10 times = 3 (at 8 times it’s about 3.4)

Desired setting = 2.4 Therefore time multiplier = 2.4/3 = 0.8 or 2.4/3.4 = 0.7 in the second case.

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Saturday, January 12, 2013 17:36