IDMT Ground-Fault Protection (ANSI 51G)
Presentation
IDMT ground-fault protection provides protection against phase-to-ground fault, which is more sensitive than protection based on phase current only. It is generally used in TN-S systems but could also be used in other earthing systems.
IDMT ground-fault protection is based on the summation of the phases and neutral current.
The IDMT ground-fault protection Digital Module provides ground-fault protection based on a High Voltage Fuse (HVF) tripping curve (I4t), providing the ability to be selectively coordinated with fuses.
Availability
IDMT ground-fault protection is available when the ANSI 51G - IDMT ground-fault protection Digital Module is purchased and installed on the MicroLogic control unit.
IDMT ground-fault protection requires an external 24 Vdc power supply.
IDMT ground-fault protection is compatible with:
-
3-pole and 4-pole circuit breakers. For details of possible configurations, refer to the table below.
-
MicroLogic 2.0 X and 5.0 X control units for IEC standard
-
MicroLogic 3.0 X and 5.0 X control units for UL standard
-
MicroLogic X control units with firmware version greater than or equal to 005.103.000. Earlier firmware versions need to be updated. For more information about firmware updates, refer to DOCA0144EN MasterPacT MTZ - MicroLogic X Control Unit - Firmware Release Notes
An External Neutral Current Transformer (ENCT) can be used for measurement of the current on neutral. For information about the installation of ENCT, consult the instruction sheet on the Schneider Electric website: NHA14388.
Operating Principle
The ground-fault current Igf is calculated by summation of the instantaneous phases and neutral current according to the circuit breaker configuration, as shown in the following table. As a result, it does not need an additional sensor to measure the ground current.
|
Circuit breaker configuration |
Igf ground-fault current |
|---|---|
|
3P |
Igf = I1 + I2 + I3 |
|
4P |
Igf = I1 + I2 + I3 + IN |
|
3P + ENCT |
Igf = I1 + I2 + I3 + IN (ENCT) |
The IDMT ground-fault protection threshold Ig sets the level of ground-fault current at which the circuit breaker detects a ground fault that can lead to a trip, based on a High Voltage Fuse (HVF) tripping curve (I4t).
The time delay tg sets the minimum length of time during which the circuit breaker carries a ground-fault within the IDMT ground-fault protection threshold Ig range.
The IDMT ground-fault protection threshold IgMax sets the level of ground-fault current at which the circuit breaker trips with the time delay tg as a constant time protection.
IDMT ground-fault protection is based on the true RMS current of phases and neutral.
In order to trip on an intermittent electrical fault, the control unit accumulates the intermittent currents in the ground-fault tripping range that do not last long enough to trigger a trip. In this case the tripping time is progressively reset at each intermittent electrical fault and may lead to shorter tripping times than those set.
Tripping Time Calculation
For this time-dependent curve I4t, the tripping time depends on the current I compared to the IDMTG Ig threshold, IDMTG IgMax limit and the IDMTG tg time delay.
The tripping time (Ttrip) is calculated using the following
equation, according to IEC 60255–151 Annex A:
As
the curve is based on an HVF tripping curve, the value of the parameters k, c, and α are fixed (k = 80, c = 0, α = 4). This leads to the following simplified
formula:
Tripping Time According to IDMT Ground-Fault tg Time Delay
The following table indicates the tripping time in seconds according to the IDMT ground-fault tg time delay, for:
-
Ig = 200 A
-
IgMax = 1200 A
| IDMT GF tg (s) | 0 | 0,1 | 0,2 | 0,3 | 0,4 | 0,5 | 0,6 | 0,7 | 0,8 | |
|---|---|---|---|---|---|---|---|---|---|---|
| Current I | Tripping time (s) | |||||||||
|
250 |
0 |
89,8 |
179,7 |
269,5 |
359,4 |
449,2 |
539,1 |
628,9 |
718,7 |
|
|
300 |
0 |
31,9 |
63,8 |
95,6 |
127,5 |
159,4 |
191,3 |
223,1 |
255 |
|
|
400 |
0 |
8,6 |
17,3 |
25,9 |
34,5 |
43,2 |
51,8 |
60,4 |
69,1 |
|
|
500 |
0 |
3,4 |
6,8 |
10,2 |
13,6 |
17 |
20,4 |
23,8 |
27,2 |
|
|
700 |
0 |
0,9 |
1,7 |
2,6 |
3,5 |
4,3 |
5,2 |
6,1 |
7 |
|
|
900 |
0 |
0,3 |
0,6 |
0,9 |
1,3 |
1,6 |
1,9 |
2,2 |
2,5 |
|
|
1200 |
0 |
0,1 |
0,2 |
0,3 |
0,4 |
0,5 |
0,6 |
0,7 |
0,8 |
|
Setting the Protection
IDMT ground-fault protection can be enabled or disabled.
The IDMT ground-fault protection settings are:
-
IDMT mode: enables (ON) or disables (OFF) IDMT ground-fault protection
-
IDMT action: trip or alarm
-
IDMT inhibit: enables (ON) the protection to be inhibited by IO module
-
Ig: IDMT ground-fault protection threshold
-
IgMax: for ground-fault current Igf above IgMax, tripping time is tg
-
tg: IDMT ground-fault protection minimum time delay
It can be set as follows: .
-
On the MicroLogic X display screen, at
-
With EcoStruxure Power Commission software (password-protected)
-
With EcoStruxure Power Device app (password-protected)
The dual settings function does not apply to the IDMT ground-fault protection. When the dual settings function is enabled, IDMT ground-fault settings are the same whether set A or set B settings are activated.
Protection Settings
Settings for MicroLogic X IEC standard
|
Setting |
Unit |
Range |
Step |
Factory setting |
Accuracy |
|---|---|---|---|---|---|
|
IDMT mode |
– |
ON/OFF |
– |
OFF |
– |
|
IDMT action |
– |
Alarm/Trip |
– |
Alarm |
– |
|
IDMT inhibit |
– |
ON/OFF |
– |
OFF |
– |
|
Ig |
A |
0.2–1 x In |
1 A |
1 x In |
+/– 10% |
|
IgMax |
A |
Ig–5 x In |
1 A |
2 x In |
+/– 10% |
|
tg |
s |
0–0.8 |
0.05 |
0.4 |
- |
Settings for MicroLogic X UL standard
|
Setting |
Unit |
Range |
Step |
Factory setting |
Accuracy |
|---|---|---|---|---|---|
|
IDMT mode |
– |
ON/OFF |
– |
OFF |
– |
|
IDMT action |
– |
Alarm/Trip |
– |
Alarm |
– |
|
IDMT inhibit |
– |
ON/OFF |
– |
OFF |
– |
|
Ig for In < 1200 A |
A |
0.2–1 x In |
1 A |
1 x In |
+/– 10% |
|
Ig for In ≥ 1200 A |
A |
500–1200 A |
1 A |
1200 A |
+/– 10% |
|
IgMax for In < 1200 A |
A |
Ig–5 x In |
1 A |
2 x In |
+/– 10% |
|
IgMax for In ≥ 1200 A |
A |
Ig–5 x In |
1 A |
3000 A |
+/– 10% |
|
tg |
s |
0–0.8 |
0.05 |
0.4 |
- |
Zone Selective Interlocking (ZSI)
ZSI does not apply to the IDMT ground-fault protection.
Predefined Events
The function generates the following predefined events:
|
Code |
Event |
History |
Type |
Latched |
Activity |
Severity |
Service LED |
|---|---|---|---|---|---|---|---|
|
0x6432 (25650) |
|
Trip |
Pulse |
Yes |
Enabled |
High |
No |
|
0x6232 (25138) |
|
Protection |
Entry/Exit |
No |
Enabled |
Low |
No |
|
0x6332 (25394) |
|
Protection |
Entry/Exit |
No |
Enabled |
Medium |
No |
Predefined events cannot be modified by the user. For general information about events, refer to Event Management.
Protection events are generated as follows:
-
The start event is generated when the protection picks up.
-
The operate event is generated when the protection time delay elapses.
-
The trip event is generated when the circuit breaker tripping voltage release (MITOP) activates.
The trip event is not generated when:
-
The optional protection is set in alarm mode
-
The optional protection is inhibited
Recommended Actions
|
Code |
Event |
Recommended actions |
|---|---|---|
|
0x6432 (25650) |
|
Reset the device or use the Power restoration assistant within the EcoStruxure Power Device app. |
Resetting a Trip Event
For information about resetting the circuit breaker after a trip due to an electrical fault, refer to the relevant document:
-
MasterPacT MTZ1 - Circuit Breakers and Switch-Disconnectors - User Guide
-
MasterPacT MTZ2/MTZ3 - Circuit Breakers and Switch-Disconnectors - User Guide