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Electromechanical Relays
By Steven McFadyen (/user-profile/userid/3)on April 2nd, 2012
Electromechanical relays have been the traditional backbone of electrical protection systems. While over recent years these
have been replaced by microprocessor based numerical devices, there are still many older electromechanical relays in
service.
Contents [hide]
1. Evolution
2. Tripping Curves
1. IEC 60255 Characteristics
2. North American Characteristics
3.
4. CDG11/16 Curves
5. GEC / English Electric / Alstom /Areva
1. Stabilising Resistors
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Evolution
Coils of wire on the rotor carry a d.c. current which
generates a magnetic field. A stator magnetic field is created
Mechanical relays developed in the 1800s were the first form of electrical protection. While still being reliable and widely
using either permanent magnets or...
used these were superseded by static relays in the early 1980s. Static relays have no moving parts (hence the name) and
operated on the basis of analogue circuitry. More recently static relays have been superseded by first digital relays and now
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numerical microprocessor based devices.
notes/entryid/41/robot-folds-towels)
The robot folding towels post below was interesting enough
at the time to post a link. Recently I’ve come across a
Tripping Curves
IEC 60255 Characteristics
couple of other interesting videos...
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I've been considering a blog on the 110 or 230 Volt issue for
(http://myelectrical.com/Portals/0/DNNArticle/Files/718/WLWimages/WindowsLive-Writer-Electromechanical-Relays_BFC6-IEC60255Curves_4.jpg) The IEC
60255 standard defines four standard current time characteristics – standard
inverse (SI), very inverse (VI), extremely inverse (EI) and long-time inverse. Each
characteristic can be calculated from:
a while. While browsing the Internet I came across a great
summary by Borat over at engineering...
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In the previous note we looked at the approach taken by the
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When thinking about watts (W) or kilowatt (kW = 1000 W) it
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At the beginning of the year China put the world's first 1,000
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(http://myelectrical.com/Portals/0/DNNArticle/Files/718/WLWimages/Windows-Live-
Standard Cable & Wire Sizes (https://myelectrical.com/
Writer-Electromechanical-Relays_BFC6-IEC60255_2.png)
notes/entryid/166/standard-cable-wire-sizes)
IEC 60228 is the International Electrotechnical
where:
Commission's international standard on conductors of
insulated cables. Among other things, it defines a...
t
= tripping time in (S)
I
= fault (actual) secondary CT current (A)
Is
= relay pick-up current setting)
TMS = time multiplier setting
Characteristic
α
K
Standard Inverse
0.02
0.14
Very Inverse
1.0
13.5
Extremely Inverse
2.0
80
Long-time Inverse
1.0
120
Relay characteristics are sometimes classified according to the tripping time at 10 times the setting current (i.e. [3s/10] - a
standard inverse curve which will trip in 3 seconds at 10 times the current setting). Tripping times for the various relays are:
Standard Inverse (SI)
[3s/10] or [1.3s/10]
Very Inverse (VI)
[1.5s/10]
Extremely Inverse (EI)
[0.8s/10]
Long Time Standard Earth Fault[13.3s/10]
North American Characteristics
(http://myelectrical.com/Portals/0/DNNArticle/Files/718/WLWimages/WindowsLive-Writer-Electromechanical-Relays_BFC6-NorthAmericanCurves_4.jpg) Current
time characteristics in North America as classified as IEEE Moderately Inverse,
IEEE Very Inverse, IEEE Extremely Inverse, US C08 Inverse and US CO2 Short
Time Inverse. These are given by:
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(http://myelectrical.com/Portals/0/DNNArticle/Files/718/WLWimages/Windows-LiveWriter-Electromechanical-Relays_BFC6-NACurves_2.png)
where:
t = tripping time in (S)
I = fault (actual) secondary CT current (A)
Is = relay pick-up current setting
TD = time dial setting (multiplier)
Characteristic
α
β
K
IEEE Moderately Inverse
0.02
0.114
0.0515
IEEE Very Inverse
2.0
0.491
19.61
IEEE Extremely Inverse
2.0
0.1217
28.2
5.95
US CO8 Inverse
2.0
0.18
US CO2 Short Time Inverse
0.02
0.01694 0.02394
Setting Example (IEC 60255)
CDG11/16 Curves
If you have the 3sec relay's trip curve, you can just multiply the time with
1.3 and divide the answer with 3. That is the time for the 1.3sec relay.
An 1000 Amp breaker protected by relay with
Standard Inverse characteristic. The relay
pick-up current value is set at 0.8, time
multiplier setting is 7 and the fault current is
8000 A. What will be the tripping time?
GEC / English Electric / Alstom /Areva
• - from the table α = 0.02, K = 0.14
Labelling the model from left to right, using number CDG31 etc. 1=C,
• - pick-up current setting = 1000 A x
2=D c=G etc.:
0.8 = 800 A
• - using the IEC 60255 equations, the
1. operating quantity (C - current, D - differential, V- voltage)
tripping time is:
2. basic movement (D - induction disc, M - balanced armature, T
- static)
3. Application (G- general or generator, E - earth, U - definite
time, F - flag, M - motor, D - directional)
4. number of units (ie CDG3x is a 3 element / unit CDG relay)
(http://myelectrical.com/Portals/0/
SunBlogNuke/2/Windows-Live-Writer/
5. characteristic (for CDG, 1= std inverse (3s), 2= long time delay, 3=very
inverse (1.55s), 4=extremely inverse (0.6s),
Electromechanical-Relays_E3C0/
6=Long Time Standard Earth Fault)
6. case size (15 different cases, A=size 1 draw out, 10 terminal etc.) image001_6.png)
7. case mounting (F=flush etc.)
8. identification (identifies rating, contact arrangement etc. 2= 'metricated')
9. suffix ('5' is for 50Hz only relays, '6' for 60Hz)
For a full list refer to publication MS/5100/2 from English Electric
We also have an online IDMT Tripping Time
calculator (http://myelectrical.com/tools/idmt-
tripping-time) .
The company history has changed over the last few years. English Electric became GEC and subsequently GEC-Alstom.
Recently the company has now been acquired by Areva.
example - CDG 34EG0022A5 is a current operated, induction disc general relay, with three extremely inverse elements and is a
50 Hz unit.
Stabilising Resistors
If current transformers are connected in a residual circuit, saturation of one or more of the transformers during transient
events may result in large spill currents. Particularly with high impedance relays, this may cause the the relay to falsely
operate. To achieve stability under these conditions, stabilising resistor|resistors are added to increase the minimum relay
operating voltage.
Rule of thumb for sizing of resistor:
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try to
drop:
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at
,
that is:
Where:
R
= stabilising resistor value
Vk
= knee voltage of CT
Ipk
= relay pickup current
Alternatively the stabilising resistor can be calculated by using:
where:
VA
= relay burden
Ir
= relay setting current
Note: The power rating of the stabilising resistor should be chosen taking into consideration the expected magnitude and
duration of the current through the resistor.
Protection (https://myelectrical.com/notes/tag/protection), Protective Devices (https://myelectrical.com/notes?
tag=protective+devices), Relays (https://myelectrical.com/notes/tag/relays), IDMT (https://myelectrical.com/notes/tag/
idmt)
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Steven McFadyen (/user-profile/userid/3)
Steven has over twenty five years experience working on some of the largest construction projects.
He has a deep technical understanding of electrical engineering and is keen to share this
knowledge.
About the author(/user-profile/userid/3)
(http://myelectricalengineering.co.uk)
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FELIX M.S. JR.
10 years ago
Best
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− ⚑
edited
This is good reference coming from a respected electrical engineer. You give good explanations
about difference of settings from various international standards.
2
U
⥅
Reply
0
− ⚑
Ujjappa
6 years ago
Sir/mam can you tell me what are materials used in electromechanical over current relay (CDG11)
1
G
⥅
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0
− ⚑
Guest
10 years ago
edited
Hi I am pretty new to this but just a quick one .. doesn't the 20s time to trip seem a bit high and
TMS is generally in the range between 0.025-1.5. If it was just for the purpose of explanation the
TMS of 7 was chosen please ignore that part.
1
⥅
Reply
0
> Guest
Steven McFadyen Mod
10 years ago
− ⚑
Thanks for the comment. The values chosen were only for demonstration. Your right that
in a real situation, you would not want to fault to be there for 20 seconds..
4
A
Reply
0
⥅
− ⚑
A Brese
6 years ago
Hi Steven. When I calculate the IEC Extremely Inverse (EI) curve using the 60255 equation at 10
times pickup value (TM = 1), i get 0.8s as per your calculation. However, when I look at your timecurrent plot, I see that 10 times pickup for the EI curve comes in at 0.6s. When I look at the timecurrent curve for a CDG34 EI relay, I see it comes in at 0.6s as well... I must be doing something
silly, but I was wondering if you were able to help please?
Thanks in advance.
0
FB
⥅
Reply
0
− ⚑
Francois Bense Potgieter
6 years ago
Hi,Steven do you perhaps have info on the English Electric VDG 11 overvoltage relay.I need the trip
time formula and the time/voltage characteristics with the TD or different time multipliers .
regards
Francois
0
⥅
Reply
0
> Francois Bense Potgieter
Steven McFadyen Mod
6 years ago
− ⚑
I've just checked and don't have a copy of the manual. Sorry. You could try the
manufacturers website, under legacy devices (this worked for me a few years ago).
12
NF
Neels Ferreira
6 years ago
0
Reply
⥅
− ⚑
Hi Steven, We at Mittal just want to make sure that we follow the cdg codes correct. Are we correct
to say,
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