Protective Measures
Protective Measures
Protective measures are designed to prevent the flow of dangerous currents through the body and protect system components from destruction. These may include measures that shut down the system completely in the event of a fault, design measures that prevent access to dangerous components and even the systematic use of harmless voltages (12-volt vehicle electrical system).
Fault types
The following types of fault occur in hybrid technology:
Short circuit
A conductive connection between two live active conductors with different voltages when no useful resistance is present in the faulty circuit. The overcurrent protective device (fuse) triggers.
Conductor fault
The conductor fault is a connection between live active conductors when a useful resistance is present in the faulty circuit. The overcurrent protective device does not trigger, but a malfunction occurs due to the continuous presence of the useful resistance (similar to a "sticking" contactor).
Ground fault
An active conductor reaches the conductive housing of an object. When contact is made, contact voltages pass between the housing and the reference potential of the energy producer. The voltage levels depend on the contact resistances and the resistance of the actual object (series connection).
Contact resistances
Contact resistances are generated in several different ways. Usually they go unnoticed at first but grow over time and eventually affect the safety and function of the overall system.
Contact resistances are caused by:
- Incorrectly seated or faulty connectors
- Corrosion resulting from moisture that enters the electrical connections
- Dirt and residues in electrical connections
- Dirty contact surfaces on ground connections or screw connections that form part of the equipotential bonding system
- Poor transitions from the line to the connector
- Cable cross sections too small
Contact resistances are connected in series with the electrical circuit and distort the voltage level on downstream components.
While contact resistances on a 12-volt system cause a voltage to decrease by only a few volts, the power transferred between high-voltage components is much higher. Contact resistances generate voltages that heat up contact surfaces considerably, the consequences of which can range from progressive damage to a risk of fire.
The isolation monitoring function of the battery manager detects any contact resistances generated around the equipotential bonding conductors, identifies them as system faults and restricts the functions of the hybrid system.
Identification of high-voltage components and high-voltage vehicles
High-voltage components
Saftey warning labels that draw attention to potential dangers are applied to all high-voltage components.
All high-voltage connections are protected and orange in color, which makes them stand out clearly from other components in the vehicle.
High-voltage vehicles
Warning signs must always be attached to hybrid vehicles residing in the workshop to draw the attention of third parties to potential dangers.
Safety: The high-voltage technician (HVT) is solely responsible for attaching warning signs to high-voltage vehicles correctly, disconnecting the high-voltage supply and starting the vehicle again after all the necessary work has been performed.
The five safety regulations
Five safety regulations must always be observed without exception before work is performed on the electrical system. Immediately after the high-voltage vehicle is driven into the workshop, the high-voltage technician must mark the vehicle accordingly (see warning sign on right). The name of the HVT responsible for marking the vehicle must appear on the sign.
NOTE:
- In a corresponding test report, the HVT must document that the vehicle has been disconnected and specify any tests performed to establish that the high-voltage supply is isolated correctly.
- See workshop literature.
1. Disconnection
The vehicle may only be disconnected by a high-voltage technician (HVT).
The following actions are required:
- Ignition off
- Remove service disconnector
- Remove the pilot line connector from the E-box
- Disconnect the high-voltage lines from the E-box
2. Securing against restart
Ensure that no third parties are able to restart the high-voltage system.
The high-voltage technician (HVT) responsible must keep all components required to restart the system such as the:
- ignition key
- service disconnector and
- pilot line connectors
in a safe location to which only he has access.
NOTE:
- Only the first three regulations in the list apply to work performed on high-voltage vehicles.
- Strict compliance with these regulations guarantees your own personal safety and the safety of others. See also "Competencies and responsibilities".
3. Checking that the high-voltage system is disconnected correctly
The high-voltage system is tested metrologically to determine whether it is isolated from the voltage supply. The following measurements are made to check that the high-voltage system is isolated:
- Voltage detector check by applying a reference voltage
- E-box measured using a suitable measuring adapter after the high-voltage cables are disconnected
- High-voltage direct-current connections on the power electronics measured using the measuring adapter
- Reference voltage checked again
After de-energized state has been established, the vehicle must be marked to indicate this by the responsible HVT using the corresponding warning sign (see bottom of page 171). The name of the HVT must be shown on this sign. The red warning sign is removed.
4. Grounding and short-circuiting
Active conductors are also connected to the ground potential and the neutral conductor. On systems with a nominal voltage of less than 1,000 V, these measures are not required.
NOTE: The simplified illustrations shown below represent the principles of the three different network types used in vehicle engineering.
5. Covering or isolating adjacent live parts
Network types
Network types define the structures of the paths that transmit electrical energy. Energy providers or even the operator of an electrical system determine the network type, which may vary depending on the region. Depending on the selected network type, different measures that prevent dangerous currents from passing through the body are available or even required. While the most common network type used in private households nationwide is the TN system, an IT network is consistently used in hybrid vehicles.
Network types are identified by an alphabetic abbreviation.
The first letter defines the grounding state of the energy producer (transformer station or high-voltage battery)
T The transformer star point in the three-conductor network is connected to ground. In terms of vehicle technology, this represents a connection of the negative terminal on the 12 V battery to the vehicle ground.
I The energy producer is isolated from the ground or a shared reference potential. In hybrid technology, all high-voltage connections are completely isolated from the rest of the vehicle.
The second letter describes the grounding state of the energy consumer.
N Conductive housings on consumers are connected to the operating ground (producer) via a protective conductor or PEN conductor.
T Conductive housings on consumers are connected directly to the ground potential or a shared reference potential (vehicle body).
The first and second letters combine to form the three common network types: TN network, TT network and IT network.
Only IT networks are used in hybrid vehicles, which includes the Cayenne S Hybrid. Instead of a connection to ground, a connection to the vehicle body is established, which can be considered isolated from ground. The different versions do not take into consideration connections between a person and ground.
TN and TT system
TN network
In the TN network, the star point of the energy producer is grounded.
The housings on the consumers are connected to the operating ground of the energy producer via the protective conductor and the PEN conductor (PEN = Protective Earth Neutral). There are two types of TN system:
- TN-C network
- TN-S network
TN-C network
In the TN-C system (C=combined), the neutral conductor simultaneously assumes the function of the protective conductor (see "Protection class 1") because the protective conductor is connected directly to the PEN conductor on the consumer and therefore connected to the operating ground as well. The disadvantage of this system comes at the moment during which the PEN conductor between the consumer and energy producer is interrupted. The connection between the protective and PEN conductors then makes available the full nominal voltage between all conductive housings and ground.
Non-functional equipment causes the system to adopt a de-energised state. If any part of the body makes contact with the housing, there is a risk of fatal injury. This type of network may therefore only be used for cable cross sections exceeding 10 mm2. It is often the site connection box on the consumer. A sealed housing and larger cross sections ensure that the configuration is correct here.
TN-S network
In addition to the neutral conductor, the TN-S system (S=separated) incorporates a separate protective conductor (green-yellow) and is standard for structures at the consumer end. Equipment from protection class 1 is connected via the shared protective conductor. Only the TN-S system offers the possibility of using a fault current protection circuit (FI protection switch) as an additional protective measure.
The advantages of the two networks
- TN-C = fewer conductors
- TN-S = safe
are combined in the TN-C-S system. The longer transmission path between the energy producer and the consumer on the TN-C system is combined with the consumer installation on the TN-S system. In the site connection box (=transmission point), the PEN conductor is divided into neutral and protective conductors.
A TN system does not mean that there is no ground in the consumer installation. Rather the "additional" system ground at the consumer end combined with the main equipotential bonding circuit is essential. In other words, the provision of a shared reference potential and connection with the grounding wire system and all other conductive parts of the building. The purpose of this configuration is to discharge potentials to ground and prevent impermissible contact voltages from developing when a fault occurs.
If the TN system were used in hybrid technology and a ground fault occurred, the connection between the housings and the reference potential (protective ground) would trigger the fuse. As a consequence, the high-voltage supply would fail during operation and this is one of the main reasons why this system is not used.
TT network
The TT network is used primarily in rural sectors and only very rarely. Unlike the TN network, conductive housings are not connected with neutral conductors (no protective ground), but instead are connected to the ground potential. The ground contact resistances must be very low so that the fuse can trigger quickly enough if a fault occurs.
IT network
In the IT network, the energy producer (high-voltage battery) is not connected to the reference potential. Appropriate measures have been taken to isolate both terminals on the high-voltage battery from the vehicle body (reference potential). Isolation must be guaranteed through the introduction of suitable design features and continuous monitoring. If the electrical system is spread over a large area, the capacitances in the lines ensure that the longer the transmission paths, the smaller the leakage resistance. As a result, larger systems cannot be integrated as an IT network.
The major advantage of an IT system is the isolation of the energy producer from the reference potential.
In a similar way to protective separation, current does not flow between the housing and the body of the vehicle in the event of a fault (ground fault). Furthermore, the fuse does not shut down the system in the same way as the TN system because a system shutdown is not necessary. The high-voltage system remains operative (1 fault system) and the hybrid vehicle can therefore visit the workshop without posing a danger.
On the Cayenne S Hybrid, the battery manager is responsible for monitoring the leakage resistance. It measures the leakage resistance by comparing the current and voltage values in the overall system. If the values are different, the battery manager assumes that an insulation fault is generating a current flow.
A detected insulation fault is indicated on the instrument cluster by a yellow warning lamp.
The HV system is switched off if a second insulation fault is detected. The warning indicator is now shown in red. This means that the vehicle can now be operated only with the combustion engine, and renewed starting is no longer possible.