In fact, even if I could get to the positive pole of the vehicle battery (ev) high voltage battery (which doesn’t work, it is completely protected against contact even when unplugged), I would not get an electric shock. But why is that?
Short answer: Because no circuit is closed.
To understand this in more detail, we need to go a bit into the basics of grid systems, from the classic low-voltage grid. This is also a chapter in our high-voltage training courses in accordance with DGUV I 209-093 .
To remember: power grid systems are used to supply electrical energy to consumers. This is known to specialists for (ev) high voltage as well as electrical specialists according to VDE.
The different types of network systems are divided into different variants of low-voltage networks. In some cases, these go beyond the necessary voltage and current-carrying lines, which are intended to fulfill purely technical functions. (ev) high voltage is a bit different, but is based on low voltage in many ways. In low-voltage electrical networks are distinguished according to the following criteria:
- Type of current, i.e. alternating or direct current system.
- Type of active conductors (for example L1, L2 and L3, or L-Plus and L-Minus).
- Type of earth connection of the system.
The choice of the ground connection is decisive here, because it significantly determines the properties as well as the safety-relevant behavior of the network. It also influences aspects of usage, which affect the following factors:
- Security of supply.
- Installation Effort.
- Maintenance effort.Maintenance downtime.
- Electromagnetic compatibility – EMC for short.
In the course of the safety-related assessment, a distinction is made between various network forms. The differences here are the number of active conductors and the termination points. In particular, the type and location of the groundings are of central importance. The basic network forms can be divided into:
- TT network.
- TN networks divided into
- TN-C network,
- TN-S network,
- TN-C-S network.
- IT network.
TT network system
To remember: In TT systems, a point is directly grounded. This is called operational grounding. In this case, the bodies of the electrical system are connected to earth electrodes, which operate electrically independently of the earth electrode for grounding the system.
This means that there is no line connection between system grounding and supply grounding.
Permissible protective devices for this are:
- Overcurrent protection device.
- Residual Current Devices (RCDs).
TN network systems
To remember: In TN systems, a point is directly grounded. The bodies of the electrical system are connected to this point via protective conductors.
According to the arrangement of the neutral conductors as well as the protective conductors, three types of TN systems can be distinguished:
- TN-C network: Throughout the system, the functions of the neutral and protective conductors are combined in a single conductor.
- TN-S network: A separate protective conductor is applied throughout the system.
- TN-C-S network: In one part of the system, the functions of the neutral conductor and the protective conductor are combined in a single conductor.
IT network system
Spoiler alert, now we get to the high-voltage stuff!
To remember: In IT systems, either all active conductors are isolated from ground or one point is connected to ground via an impedance.
In the event of an insulation fault, therefore, only a small fault current, essentially caused by the system leakage capacitance, can flow; the upstream fuses do not respond. However, the voltage supply is maintained even in the event of a single-pole, direct ground fault. The bodies of the electrical installation are either
- individually grounded or
- grounded together or
- connected together with the grounding of the system.
The following protective devices are permissible for IT networks:
- lsowächter or IMD.
- Overcurrent protection devices.
- RCD or residual current device in individual circuits.
The great advantage of the IT network is that a single fault does not cause the system to shut down and, thanks to the isowatch, it is also detected at an early stage.
IT systems can be found, for example:
- In the operating room at the hospital.
- In the railcar of the railroad.
- As explosion protection in mining.
- For sensitive industrial production.
- For photovoltaic systems on the generator side.
- In adapted form in the high-voltage system of electric vehicles. (Yes, right here!)
In summary, the following can be said about the IT system:
- A first insulation fault does not lead to disconnection by a fuse.
- A first insulation fault also does not lead to disconnection by a residual current circuit breaker.
- An isowatch detects insufficient insulation resistance.
- An insulation fault should be cleared as soon as possible before a second insulation fault occurs on another active conductor, which would cause the network to fail.
How every specialist high voltage (ev) high voltage needs to know: The high-voltage network in the vehicle is an IT-like network system
Wait, just an IT-like network? Why is the network in the electric vehicle only IT-like and not a real IT network?
Well, the “T” stands for earth. And unless the high-voltage vehicle is pulling a one-meter-deep plow behind it, it is not grounded. Therefore, the high voltage specialist connects all housings of all high voltage components to the vehicle body via equipotential bonding, since the mass of the vehicle body is used instead of the ground. This means that we do not have a real earth or a real IT network. And with the vehicle ground as the grounding burr, the (ev) high voltage vehicle is simply an IT-counting network.
So if someone were to touch the positive terminal of the high-voltage battery, they would not complete a circuit in an IT-like network. Of course, this should NOT be done. And the second mistake is still dangerous! Work on the HV (ev) system is not permitted without the appropriate qualification as a specialist high voltage (ev).
PS: Our recommendation here: There are also courses offered by various providers, such as the aforementioned course “Specialist high voltage (ev)” or also the small course for the specialist instructed person for high voltage. More information about this can be found on our homepage
We offer various open seminars as well as customer-specific in-house seminars.
Our free(REALLY free, even WITHOUT having to provide an email address!) paper “6 Things You Need to Know About High-Voltage Qualification of Your Employees in Advance” can be accessed here (click) .
If you want to know more about the different roles, especially those of the responsibilities and especially those of the CRES and their interaction, I recommend our publications, for example the audio book “The chief responsible electrical specialist: CRES structure and operational electrical safety for entrepreneurs, specialists and managers”. Information and sources of supply can be found on the usual audio book portals as well as on the homepage tcs-engineering.de