Analysis of power battery system safety design provided by electronic
enthusiasts: Pack equipotential connection technology. Electric shock protection
is an important part of the electrical safety design of power battery systems.
Generally speaking, it can be achieved through two methods: one is direct
contact protection, Such as insulation design, screen protection (barrier/shell,
IPXXB/IPXXD, etc.); second, indirect protection, including equipotential
connection, electrical isolation (electrical clearance, creepage distance). Here
we talk about some understanding and understanding of equipotential. Electric
shock protection is an important part of the electrical safety design of power
battery systems. Generally speaking, it can be achieved through two methods: one
is direct contact protection, such as insulation design, screen protection
(barrier/shell, IpXXB/IpXXD, etc.); It is indirect protection, including
equipotential connection and electrical isolation (electrical clearance,
creepage distance). Here we talk about some understanding and understanding of
equipotential.
What is equipotential bonding?
In electrical terminology, equipotential bonding is also called protective
grounding. The book "Lightning and Lightning Protection Engineering" defines
equipotential as follows: "Equipotential bonding is to connect all metal objects
in and near the building, such as steel bars in concrete. , water pipes, gas
pipes and other metal pipes, machine base metal objects and other large buried
metal objects, cable metal shielding layers, the neutral line of the power
system, and the building's grounding wire are all connected by electrical
connections (welding Or reliable conductive connection) makes the metal objects
inside the entire building a good equipotential body."
In the national standard GB/T18384-3: 2015 "Safety Requirements for
Electric Vehicles Part 3", equipotential connection (potential equalization) is
defined as: minimizing the potential difference between exposed conductive parts
of electrical equipment.
Why is equipotential bonding required?
In the process of continuous development and updating of electrical safety
technology, people have noticed that a large number of electrical accidents are
caused by excessive potential differences. For example, lightning casualty
accidents are caused by the tens of thousands of volts generated by lightning
directly applied to the human body and the body. The huge potential difference
between the earth and the earth produces a large instantaneous current, causing
people who are struck by lightning to be injured or killed due to respiratory
arrest or cardiac paralysis. Compared with lightning strike accidents, more
people around the world are injured or killed due to electric shocks from civil
or industrial electricity. The principle is the same as lightning strike
accidents. It is due to the huge potential difference generated by charged
objects in different parts of the human body, which in turn causes serious
injury.
The international community attaches great importance to the role of
equipotential bonding. It is very necessary for electrical safety, lightning
protection, and the normal operation and safe use of electronic information
equipment. After equipotential connection, fault voltage in the system power
line can be prevented from causing electric shock accidents. At the same time,
it can reduce the probability of potential difference, arc, and sparks, and
avoid electrical fire accidents and personal electric shock accidents caused by
ground faults.
The main functions of equipotential bonding are as follows:
To prevent personal electric shock: Make a good metal connection between
the uncharged metal conductor part of the electrical equipment during normal
operation and the ground electrode to protect human safety and prevent personal
electric shock.
Ensure the normal operation of the electrical system: The grounding of the
power system is generally the neutral point grounding. The grounding resistance
of the neutral point is very small, so the potential difference between the
neutral point and the ground is close to zero.
Prevent the hazards of lightning strikes and static electricity: Static
electricity induction and electromagnetic induction will be generated during
lightning strikes. Static electricity caused by friction during the production
and transportation of materials may cause electric shock or fire hazards.
In electric vehicle products, if the maximum voltage of the entire battery
pack exceeds 60V (DC), it has exceeded the safe voltage range of the human body,
and equipotential connections must be made to ensure safe use.
In the case of equipotential bonding, even if the insulation between the
positive or negative electrode of the battery pack and the battery pack case
fails due to a fault, since all exposed metal parts on the vehicle have reached
the same potential through equipotential bonding, the human body is exposed to
these When using metal parts, no current will be generated, the human body is
still safe on the vehicle, and no electric shock accident will occur.
What are the equipotential bonding standards relevant to electric
vehicles?
①GB/T18384-3 Safety Requirements for Electric Vehicles Part 3: Personnel
Electric Shock Protection
②EN1987-3Electricallypropelledroadvehicles-Specificrequirementsforsafety-part3：protectionofusersagainstelectricalhazards
③ISO6469-3 Electrically propelled roadvehicles-SafetyspecificaTIon-part3:
protectionofpersonsagainstelectricshockGB/T18384-3 The design requirements and
test requirements for equipotential connection are clearly specified in
standards 6.3.1 and 6.9.
How to design and inspect equipotential bonding?
For the verification of equipotential bonding, the test method given in
GB/T18384-3:2015 is in part 7.4 of the standard:
The test diagram and examples of GB/T18384-3:2015 are shown in the figure
below:
The equipotential bonding requirements set by some customers are more
stringent than GB/T18384-3:2015 and ISO6469-3, requiring that the impedance of
the equipotential bonding must be less than 0.01 ohm.
So, how to design the equipotential connection for the power battery
system?
Design ideas:
Design principle:
Design:
First of all, it is required that the casing of the battery box must be
connected to the ground of the vehicle (vehicle casing) at an equal potential,
either by ground wire connection or by thick bolts, depending on the overall
design of the vehicle.
Key points:
1) Ground wire connection, the color of the ground wire is black;
2) Thick bolt connection
Secondly, all accessible conductive metal parts on the battery box shell
(such as covers, brackets, water-cooling tubes, etc.) must be connected to the
shell at equal potentials. This can be done through welding, crimping, bolting,
etc. realized in a way. If the equipotential connection is achieved by crimping
or bolting, the contact surface cannot be painted or insulated, otherwise the
contact resistance will be very large and cannot meet the requirements. There
are also corresponding specification requirements for the type and torque of
bolts for equipotential connection.
Key points:
1) Welding, welding reliability;
2) During crimping, the contact surface cannot be painted or insulated,
otherwise the contact resistance will be large and the requirements cannot be
met;
3) For bolted connections, the contact surface cannot be painted or
insulated, otherwise the contact resistance will be large and the requirements
cannot be met;
Type and torque have corresponding specifications.
Thirdly, the conductors used for equipotential connection (such as
grounding wires, etc.) are required to be black in color to facilitate
identification during maintenance and disassembly. The cross-sectional area of
the conductor and the area of the contact surface used for equipotential
connection must be guaranteed to be no less than the cross-sectional area of the
high-voltage wire harness. This is mainly because when the insulation fails,
high-voltage current may flow through the equipotential loop. If the
cross-section of the equipotential connection is If the area is not large
enough, it is likely to generate heat due to overflow and cause a fire.
Key points:
1) The conductor cross-sectional area and contact surface area used for
equipotential connection must be guaranteed to be no less than the high-voltage
wire harness cross-sectional area
2) The connection points need special treatment to avoid corrosion caused
by the potential difference between different materials.
Of course, when designing products, the technical implementation of
equipotential connection will not be limited to the technical requirements of
customers. Sufficient measures and means should be taken according to the
specific conditions of the product to ensure the requirements for potential
connection, thereby ensuring the safety of the product. .
Here are two points to emphasize:
(1) Some colleagues are easily confused about equipotential and grounding.
Here is an explanation:
Common points: The connection resistance must be small enough so that there
is no potential difference between the two connected conductors. At the same
time, there are also requirements for the color of the surface of the wire body
(the color requirements are different in different fields);
Difference: Grounding does not strictly require the current-carrying
capacity of the ground wire, while equipotentiality has clear current-carrying
requirements for connecting wires: greater than but not less than the
cross-sectional area of the main power harness, that is, the current-carrying
capacity is equal to or greater than the main power harness.
(2) Some electric buses use insulating pads to connect the electrical box
and the body chassis, which is risky. Usually, one end of the BMS insulation
monitoring subsystem is connected to high voltage (high voltage positive or high
voltage negative), and the other end is connected to 24V ground (ie, body
chassis). The BMS determines whether the insulation has failed by collecting the
potential difference between the high voltage positive/negative and the body
chassis. When water enters the electrical box and the insulation resistance
between the electrical box and the high voltage fails,
Since an insulating pad is used between the electrical box and the car
body, and the box is not connected to the BMS, the BMS can only detect the
insulation failure between the high voltage and the car body, but cannot detect
the insulation failure between the box and the high voltage.
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