Selecting and deciding on circuit breakers
made easy with IEC specifications
Er. Say Leong LIM
2019-11-29
Leading Global Electrical Equipment & Integrated Supplier
Purpose of my talk
• Show the differences between the IEC standard specifications for circuit breakers
• Stress the importance of differentiating between the users – important for safety
• Explain the approach to select the circuit breakers in terms of load and fault current
• Explain the co-ordination requirement
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Miniature Molded case Air-break
circuit breaker circuit breaker circuit breaker
MCB MCCB ACB
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Purpose of circuit breakers
Protection of people and property by
watching the electric circuit condition for
overloading and short circuit
Global Standard Specifications - IEC
Circuit breaker
A mechanical switching ACB
device capable of
making, carrying and
breaking currents under
normal circuit conditions;
and also making,
MCCB
carrying for a specified
time and breaking
currents under specified
abnormal circuit MCB
conditions such as those
of short-circuit.
Function of circuit breaker
Utility supply
• Isolate for safety MV/LV transformer
• Disconnect when
Main LV switchboard
danger arises
LV Power
switchboard
LV final
distribution
International Standards Developers
The International Electrotechnical Commission website
states:
“The International Electrotechnical Commission
(IEC) is the leading global organization that prepares
and publishes international standards for all electrical,
electronic and related technologies. These serve as a
basis for national standardization and as references
when drafting international tenders and contracts.”
IEEE and IEC harmonize their standards, where possible
International Electrotechnical Commission
• IEC - the leading global organization that prepares and
publishes international standards for all electrical, electronic
and related technologies.
• These serve as a basis for national standardization and as
references when drafting international tenders and contracts –
like for CIRCUIT BREAKERS
• IEC 60898 for circuit breakers up to 440V, 125A and 25 kA.
Circuit breakers for use by uninstructed people and not to be
maintained
• IEC 60947-2 for circuit breakers up to 1000V, for whatever the
rated current and short-circuit. Circuit breakers for use by the
instructed people and to be maintained.
Global Standard Specifications - IEC
• Use by uninstructed person • Use by instructed person
• Not meant to be serviced • Needs to be serviced
Current-limiting MCB
Values of peak cut-off current and let-through energy
Ip
I
Prospective short-circuit current
Icc
Limited short-circuit current
Prospective
peak value I2t
Limited
peak
Specific let-through value
energy
t
Icc
MCB of UL 489 vs IEC 60898
Current-limiting MCB vs Zero point extinguishing MCB
• Current limiting type of MCB to IEC 60898
t ms would normally passed to UL 489.
• On the contrary, zero point extinguishing
type MCB to UL 489 would not necessarily
t ms
3 8.3 passed to IEC 60898. Very likely can only
passed to IEC 60947-2.
t ms
• Between IEC 60898 & IEC 60947-2, for the
same duty IEC 60898 is more severe, in
the layman term.
t ms
8.3
Engineered for uninstructed person safety
IEC 61439-3 - distribution boards intended to be
used by ordinary, uninstructed people and for not
being maintained.
IEC 60898-1 - MCB at 50/60 Hz, voltage not
exceeding 440 V, current not exceeding 125 A and a
rated short-circuit capacity not exceeding 25 kA.
IEC 61008-1 IEC 60898-1 Designed for use by uninstructed people and for not
being maintained.
IEC 61439-3
IEC 61008-1 – RCCB voltage not exceeding 440 V at
50/60Hz and current not exceeding 125 A, intended
principally for protection against shock hazard.
IIEE – The Fundamental Canons
1.To accept responsibility in making sound engineering decisions consistent with the safety, health and welfare of the public, and to disclose
1.To
promptlyaccept responsibility
the factors that in making
might endanger the public sound engineering decisions consistent with
or the environment;
the safety,
2.To avoid health
real or perceived andof interest
conflicts welfare wheneverof possible,
the public, and
and to disclose toto affected
them disclose partiespromptly the factors
when they do exist;
that might endanger the public or the environment;
3.To be honest, objective, impartial and realistic in stating claims or estimates base on available facts
4.To prevent graft and corruption in all its forms
2.To avoid real or perceived conflicts of interest whenever possible, and to
5.To improve the understanding of technology and its advancement, its appropriate application, and potential consequences;
disclose them to affected parties when they do exist;
6.To maintain and improve our technical competence and to undertake technological tasks for others only if qualified by education, training or
experience, or after full disclosure of pertinent limitations
3.To be honest, objective, impartial and realistic in stating claims or estimates
7.To seek, accept, and offer honest and constructive criticism of technical work; to acknowledge and correct errors; and to credit properly their
base on available facts
contributions;
8.To treat fairly all persons regardless of race, gender, disability, age or national origin
9.To avoid injury to other’s life property, reputation or livelihood by false or malicious actions; and
10.To assist colleagues and co-workers in their professional development and to support them in following this Code of Ethics.
Philippines need to
consider circuit breakers
and enclosures to make
it safe for
uninstructed people,
the Consumers
IEC 60947-2 for the instructed
Circuit-breakers voltage does not exceed 1 000
V a.c. or 1 500 V d.c.
Additional requirements ACB
• integrally fused circuit-breakers.
• provide earth-leakage protection - Annex B
• circuit-breakers with electronic over-current
protection - Annex F
• circuit-breakers for IT systems - Annex H
MCCB
The requirements for circuit-breakers for the
protection of wiring installations in buildings
and similar applications, and designed for use
by uninstructed persons, are contained in
IEC 60898. MCB
The requirements for circuit-breakers for
equipment (for example electrical appliances)
are contained in IEC 60934.
Think enclosures
Switchboard – safety and reliable service
Circuit breakers
Objective
▪ Fundamental requirement to prevent
danger to person and property
How
Every live conductor
▪ Be alert all time to operate automatically on needs to be protected
overload or fault current in the event of:
Requirement
▪ Overload current; and
▪ Adequate breaking capacity
▪ Operate safely without danger to person & ▪ Fault or short circuit current.
property in vicinity
▪ Ready and easy restoration
Illustration by products
NA8
NM8LE NM8 NM8 NM8
NZ7
NB1LE NB1
NC8 CJ19 NU6 NB1 NM8
NS2 NJR2 NVF2
NR8
NWC5 NCK3 NCH8 NB1
NBH8LE NBH8
motor motor motor motor motor
Circuit breaker – safe protection
Primary function is overcurrent protection
Utilities supply
❑ Protect cable and MV/LV transformer
components from damage
Main switchboard
due to
▪ overload; or
▪ short circuit. Power
switchboard
❑ Provide safe isolation.
❑ Protect against electric shock.
Final or branch
Protect distribution
users
Basic overcurrent protection
Time (s)
Supply ~
IB ≤ In ≤ Iz
I2 ≤ 1.45 Iz
Cable
Cable MCCB
k2S2 = I2t
Cable
MCCB
S
t=k
I
MCCB protects the cable by
tripping before cable burns off.
Load I(A)
Short circuit selection
For short-circuit protection, the Utilities supply
circuit breaker at the point of MV/LV transformer
installation should have a short-
50kA or more Main switchboard
circuit breaking capacity that is 50kA
sufficient to withstand the stresses
likely to occur.
Power
switchboard
Final or branch
distribution
Co-ordination
The circuit-breaker at the point of Utilities supply
installation could have a short- MV/LV transformer
circuit breaking capacity that is
65kA or more Main switchboard
insufficient to withstand the 65kA fault
stresses likely to occur. 36kA rated CB
Then add another circuit-breaker at
the upstream. Power
50kA fault switchboard
Sometimes known as cascading,
or back-up protection.
Final or branch
distribution
Benefits of co-ordination
A lower breaking capacity is permitted if another protective device or devices having the
necessary breaking capacity is installed on the supply side.
In this situation, the characteristics of the devices shall be co-ordinated so that the energy let-
through of these devices does not exceed that which can be withstood, without damage, by
the device or devices on the load side.
When selecting circuit-breakers, consider the maximum let-through energy specified by the
manufacturer of the device, to ensure proper co-ordination.
The optimization of using circuit breakers to its full potential for the benefit of:
• Safe protection
– Proper discrimination
– Proper back-up protection
• Minimum use of space
• Easy installation design and selection.
As illustrated in IEC 60947-2
Discrimination Back-up Discrimination Back-up
By current By let-through energy
Co-ordination – The selection
Utilities supply
Example of discrimination and
back-up values given by MV/LV transformer
manufacturer C2 65kA Main switchboard
C1 36kA
Power
Discrimination C2 65kA switchboard
C1 36kA 14kA
Final or branch
Back-up C2 65kA distribution
C1 36kA 65kA
Level of discrimination
Utilities supply
Let us consider C2 and C1.
MV/LV transformer
Discrimination C2 65kA
C1 36kA 14kA
C2 65kA Trip Main switchboard
C1 36kA Trip
Fault at Fault value C2 C1
Z
W 5kA ON Trip
X 10kA ON Trip Y
Y 14kA Trip Trip X
W
Z 36kA Trip Trip
Co-ordination – The selection
Utilities supply
When selecting circuit-breakers,
consider the maximum let- MV/LV transformer
through energy specified by the C2 65kA Main switchboard
manufacturer of the device, to
C1 36kA
ensure proper co-ordination.
Power
Discrimination C2 65kA switchboard
C1 36kA 14kA Selectivity
Final or branch
Back-up C2 65kA distribution
C1 36kA 65kA
Back-up potection
Utilities supply
The combined breaking of the
fault current in the circuit by C2 MV/LV transformer
and C1 will have the capability C2 65kA Main switchboard
to break 65kA
C1 36kA
Back-up C2 65kA Power
switchboard
C1 36kA 65kA
Final or branch
distribution
Thanks
