Long-Time Protection

Presentation

Long-time protection on MicroLogic 5, 6, and 7 trip units protects electrical distribution applications against overload currents. It is identical for MicroLogic 5, 6, and 7 trip units.

Operating Principle

Long-time protection is I²t IDMT (Inverse Definite Minimum Time):

It incorporates the thermal image function.
It is set with the Ir pickup and as the tr trip time delay.

The long-time protection settings are:

Ir: long-time protection pickup
tr: long-time protection time delay

Setting the Long-Time Protection

Set the Ir pickup as follows:

With the MicroLogic trip unit Ir dial to preset the value and the keypad to fine-tune the value
With EcoStruxure Power Commission software (password-protected)
By sending a setting command using the communication network (password-protected)

Set the time delay tr as follows:

With the keypad on the MicroLogic trip unit
With EcoStruxure Power Commission software (password-protected)
By sending a setting command using the communication network (password-protected)

Ir Pickup Setting Values

The long-time protection tripping range is 1.05–1.20 Ir according to standard IEC/EN 60947-2.

The default Ir pickup setting value is the maximum dial position In.

The following table shows the Ir pickup value preset on the dial:

In rating	Preset values of Ir (A) depending on the trip unit In rating and the dial position
40 A	18	18	20	23	25	28	32	36	40
100 A	40	45	50	55	63	70	80	90	100
160 A	63	70	80	90	100	110	125	150	160
250 A	100	110	125	140	150	175	200	225	250
400 A	160	180	200	230	250	280	320	360	400
630 A	250	280	320	350	400	450	500	570	630

The accuracy range is + 5%/+ 20%.

Use the keypad to fine-tune the setting, in increments of 1 A:

The setting range maximum is the preset value of the dial.
The range minimum is 0.9 times the minimum preset value (for the 400 A rating, the setting range minimum is 100 A).

Example: A MicroLogic 5.2 trip unit rated In = 250 A is preset by the dial at 140 A:

The minimum preset value is 100 A
The keypad fine-tuning range is 90–140 A

tr Time Delay Setting Values

The setting value displayed is the value of the trip time delay for a current of 6 x Ir.

The default tr time delay setting value is 16 (maximum value) that is, 16 s at 6 x Ir.

The following table shows the value of the trip time delay (in seconds) according to the current in the load for the setting values displayed on-screen:

Current in the load	tr Trip time delay per setting value
Current in the load	0.5	1	2	4	8	16
1.5 x Ir	15 s	25 s	50 s	100 s	200 s	400 s
6 x Ir	0.5 s	1 s	2 s	4 s	8 s	16 s
7.2 x Ir	0.35 s	0.7 s	1.4 s	2.8 s	5.5 s	11 s

The accuracy range is -20%/+0%.

Thermal Image

The trip unit uses the calculation of a thermal image to evaluate the conductor heat rise and precisely monitor the thermal state of the conductors.

Example:

Comparison of the heat rise calculation without thermal image (diagram A) and with thermal image (diagram B):

0 Instantaneous current (cyclical) in the load

1 Conductor temperature

2 Current calculated without thermal image (diagram A), with thermal image (diagram B)

3 Long-time protection pickup: Ir

Trip unit without thermal image: On each current pulse, the trip unit only considers the thermal effect on the pulse under consideration. No tripping occurs despite the build-up in conductor heat rise.
Trip unit with thermal image: The trip unit adds the thermal effect of successive current pulses. Tripping occurs based on the actual thermal state of the conductor.

Conductor Heat Rise and Tripping Curves

Use the analysis of the equation of heat rise in a conductor, through which a current I runs, to determine the nature of physical phenomena:

For low- or medium-intensity currents (I < Ir), the conductor equilibrium temperature (for an infinite time) only depends on the current quadratic demand value. The limit temperature corresponds to a limit current (Ir pickup for trip unit long-time protection).
For low overcurrents (Ir < I < Isd), the conductor temperature only depends on the I²t energy provided by the current. The limit temperature is an I²t IDMT curve.
For high overcurrents (I > Isd), the phenomenon is identical if the I²t function of the short-time protection is active (I²t ON).

The following figure (in double log scales) represents a heat rise curve A (for an equilibrium temperature θ) and a tripping curve B (for the limit temperature θL):

1 Low intensity current zone

2 Low overcurrent zone

Thermal Memory

MicroLogic 5, 6, and 7 trip units incorporate the thermal memory function which ensures that the conductors are cooled even after tripping. Cooling lasts for 20 minutes before or after tripping.