1. Earthing systems, Harmonics Currents and GFP

2. Contents Basic Principle 3
Earthing System Technique and Suitable Devices 6 o IEC o NEC and GFP
GFP Technique 19
Harmonics and Neutral conductor 27 o network studies o Multi-source and Multi-grounding o conclusion NEC

3. Basic Principle
Definition n The 3 phases n The Neutral conductor N n The Protection conductor PE
Role n Active conductors: 3 phases + Neutral Normal current in these conductors n Protection conductor PE Insulation Fault current
Measuring this fault current is the principle of Earthing System
E56885 L1 L2 L3 N PE

4. Basic Principles PE conductor
The A point (star point of LV transformer) can be: o solidly grounded o isolated o linked with an impedance to the PE
Fault current in A, the vectorial sum of the current is equal to 0 SIph + IN+ I PE = 0 o 2 ways to measure If ( =IPE) o don’t forget the Neutral current
E56886 L1 L2 L3 N A PE

5. Basic Principles Main Ideas
Measuring the current in the PE conductor is the principle of the Earthing System Techniques
Any other Normal current mustn’t flow in the PE (especially the Neutral current) in order to avoid the tripping nuisances

6. Earthing System Techniques
The target n Protection of people n Protection against fire risks
The economical constraints n Continuity of the power supply n Avoiding tripping nuisance due to the EM Effects
IEC and NEC standards have got the same targets and the same constraints
E56887

7. Earthing system IEC 60364 TT system
Definition
n The Neutral point of LV transformer is directly connected to an earth electrode
n The exposed conductive parts of the installation are connected to an electrically separate earth electrode
E56888 L1 L2 L3 N PE Rn Ru

8. Earthing system IEC 60364 TT system
Earth fault study
The fault current generates a dangerous touch voltage Uc > UL
The SCPD is usually not suitable for eliminating this type of fault (If = a few A eg from 1 to 10 A)
Earth Leakage Relays (ELR) must be used (Setting at 300 mA to 10 A)
E56889
Uo = 230 V L1 L2 L3 N
400V / 230V
If =
11,5A
Exposed conductive metallic part
Uf = 115 V Rn
10 W Ru
10 W

9. Earthing system IEC 60364 TN system
Definition
The star point of the LV transformer is directly connected to an earth electrode (source earth)
The exposed conductive parts of the installation are connected by the PE to the same earth electrode
E56890 L1
L2 L3 N PE

10. Earthing system IEC 60364 TN-S system
Definition
The PE and Neutral conductor are separate
E56890 L1
L2 L3 N PE

11. Earthing system IEC 60364 TN-C system
Definition
The PE and Neutral conductor are common = the PEN
E56892 L1
L2 L3
PEN

12. Earthing system IEC 60364 TN system
Earth fault study
Fault current is equal to a Phase/Neutral short-circuit
The fault current generates dangerous touch voltage
Tripping of the circuit breaker by Short Time Protection or Magnetic
E56893 L1
L2 L3 N PE

13. Earthing system IEC 60364 IT system
Definition
The star point of the LV transformer is not connected to an earth electrode
The exposed conductive parts of the loads are connected by the PE conductor to a common earth electrode
E56894 L1
L2 L3 N PE

14. Earthing system IEC 60364 IT system
Earth Fault Study
There isn’t a fault current n The fault is not dangerous but it must be signaled by PIM
Principle of PIM
Injection of current, n Measurement of IR (*). (*) Insulation Resistance
E56895 L1
L2 L3 N PE

15. Earthing system IEC 60364 IT system
Principle of the FTD (*)
Detection of fault current
Type of FTD (*):
Movable tracking n Fixed tracking (*) Fault Tracking Device
E56896 L1
L2 L3 N PE

16. NEC / US Installation Standard
Defines IT - or IG (Insulated Grounding) - System and TN-S - or SG (Solidly Grounded) - System
TN-S details o Neutral conductor is not broken
the protective conductor PE is achieved by bonding the metallic parts of equipment o protection against fire is achieved by GFP

17. NEC / US Installation Standard
Example of an installation complying with the NEC
E56897
Protection against fire o the NEC imposes a maximum protection sensivity of: 1200 A

18. Earthing System conclusions
IEC NEC
Earthing TN-C TN-S TT IT idem System TN-S
I fault High High Low Very low High
Protection SCPD SCPD RCD No problem SCPD of people
Fire Forbidden Not Recommended Recommended GFP Protection recommended with RCD with RCD
TT and IT Systems naturally manage the Fire Risk by limiting the fault current

19. Ground Fault Protection GFP
GFP types
External CT installation o measurement of all Currents (don’t forget the Neutral)
GFP co-ordination o with the CB Magnetic or ST tripping o with the other GFP

20. GFP Types
Residual Sensing: vectorial sum of the currents in the phases and the Neutral
E56898 L1 L2 L3 N
Source Ground Return: single sensor placed on the Neutral earth link
E56899 L1 L2 L3 N
Zero Sequence: - for low current detection - single sensor placed all live conductors
E56900 L1 L2 L3 N

21. External CT installation
3 poles CB on 3 phases + N: on T option, the External CT must be compulsory both
installed on primary and o connected on secondary according to this diagram
E56901

22. GFP coordination
Upstream GFP must be coordinated with any downstream Short Circuit Protection Device - SCPD - or GFP
Discrimination types o current and Time discrimination with the respect of - asymptote at 1200A A / 1s point o logic (ZSI)
E56902
GFP
upstream
GFP
downs-
tream

23. Upstream GFP / downstream SCPD
Discrimination rules
E56903
Down-
stream
short
delay
fuse 2
Upstream
GFP 1
D I
30 %
I down-
I up- T I
step 2
step 1
Fuses o GFP sensibility setting up > 10 to 15 times down. fuse rating o GFP I²t function
Circuit breakers o GFP sensibility setting up > 1,5 times down. CB short time setting up o GFP short delay setting up > down. CB breaking time setting

24. Upstream GFP / downstream GFP
I down-
stream
I up- T I
1200 A
3000 A
30 % 1s
step 2
step 1
Discrimination rules
Ampermetric o up. GFP sensibility setting up > 30% down. GFP senstivity setting up
Chronometric o up. GFP short delay setting up > down. SCPD breaking time setting o Dt1 (GFP1) > D t2 (GFP2) + dt2 (SCPD2)

25. Fluorescent lighting Electrical diagram
capacitive impedance for harmonics current
Is current
THDI = 53%
Harmonics breakdown
E56914
E56915 Ph Is FL T N
E56916

26. Computer power supply Electrical diagram
single phase loads high level of harmonics
Is current
THDI = 115%
Harmonics breakdown
I3=85%, I5=65%, I7=40%, I9=20%
E56891
E56913 Is C T
E56917

27. Harmonics current Their effects
On the equipment: o capacitor, transformer, motor, alternator, … o overload and overheating, loss of the available power, added vibration
On the installation E M disturbance
On the Neutral conductor cumulating of harmonic 3 and their multiples
E56905

28. Harmonics current conclusion
IEC NEC
Earthing TN-C TN-S TT IT idem System TN-S
EM Forbidden Good Very good Very good Good disturbances PE and But be careful No problems No problems But be Neutral are avoiding PE even if Neutral even if Neutral careful together and Neutral to and PE are in and PE are in avoiding (PEN) be in contacts contacts contacts PE and Neutral to be in contacts
There isn't any fault current in TT and IT System even if the PE and Neutral are in contact

29. Application Study of network one source / two sources o with harmonics
TN- S with Neutral conductor not broken (according to NEC standard) o with GFP
E56912

30. Harmonics current / Earth fault (one source)
Harmonics current without earth fault no fault current measured by GFP
Harmonics current and earth fault the right fault current is measured by GFP PE
Service ground
GFP
Ground bus N
E56906
Service ground PE
GFP
Ground bus N
E56907

31. Harmonics current / Earth fault (two sources or more)
Both the main circuit breakers are closed, the bus coupler is open
E56907
GFP
GFP
Neutral PE
Ground bus
Loads
Service ground
Service ground

32. Harmonics current / Earth fault (two sources or more)
Harmonic current without earth fault a "false" earth fault current is measured, harmonic current are present throughout the metallic frames
E56909
GFP
GFP
Neutral PE
Ground bus
Loads
Service ground
Service ground

33. Harmonics current / Earth fault (two sources or more)
Earth fault with / without harmonic current the "right" earth fault current is not measured by the "right" GFP, a "false" earth fault current is measured by the other one this current may be present throughout the metallic frames
E56910
GFP PE
Service ground
Ground bus
Neutral
Loads

34. How to improve Network dependability ?
Dealing with the problem by using particular GFP diagrams (Modified GFP, for instance)
Avoiding to have the problem by using a 4 poles switch on the bus coupler (to break the Neutral)
E56911
GFP PE
Service ground
Ground bus
Neutral
Loads

35. How to improve network dependability ?
Breaking the Neutral conductor
Is the more efficient solution
Is the sole solution to avoid EM effects
E56911
GFP
GFP
Neutral PE
Ground bus
Loads
Service ground
Service ground