Last edit: 30/06/2023
IT Systems: the basics
In IT Systems, the neutral of the distribution transformer is ungrounded.
This electrical system is used when the disconnection of the supply, in case of a fault, generates more risks than keeping the energy. For example, in an hospital operation room, let’s suppose that while the doctor is in operation an electrical equipment has a fault; the disconnection of supply will have much worse consequences than keeping the electrical energy in the room.
The reason for not grounding the neutral is exactly to hamper the current from flowing through the human body in case it touches the faulty equipment. If you have in mind a TT or TN system, a person may be electrocuted because the current, flowing through the body, finds a way back to the source: the grounded neutral of the MV/LV transformer. If the neutral point is not grounded, there is no way back to the source!
For that reason, the standard allows not to cut energy, however the fault must be highlighted since people must realise they are working in an environment where an electric fault is present. Hereafter are the standard language:
[IEC 60364-4-41] 411.6.3 In IT systems the following monitoring devices and protective devices may be used:
- insulation monitoring devices (IMDs);
- residual current monitoring devices (RCMs)
- insulation fault location systems (IFLS);
- overcurrent protective devices;
- residual current protective devices (RCDs).
People may wonder why the standard worries about a fault which is harmless, apparently. The reason for that is due to the risk raising in case of a second fault on another phase. In case of a three phase system, the first fault may be on phase L2 and the second on L3. In this situation, if a person touches both faulty equipment, he is subject to an electric shock of 400 Vac (in Europe).
How to avoid that? Very simple: all exposed conductive parts must be bonded together!
IT Systems: how safety is guaranteed
The monitoring device is the first condition to reduce the risk. The second is the bonding of all exposed conductive parts. The reason is that if they are all bonded together, in case of a first fault on, let’s say, L1, when the second on L3 happens, the latter becomes like a short circuit.
The short circuit current triggers the short circuit protection (circuit breaker for example) to open and therefore to interrupt the energy to the equipment.
So far, so good: IT systems are safe because:
- The neutral point is not grounded and therefore the current that goes through the body that touches a faulty equipment does not find a way back to the source: the person is safe!
- In any case, the fault is detected by, for example, an Insulation Monitoring Device. The picture shows an example of an IMD: in case of no faults the three lamps are on. In case of a fault to ground on L2, the middle lamp will switch off. Of course that is a “didactical method”, just to explain the concept.
All exposed conductive parts are connected together so, in case of a second fault on another phase, a short circuit protection will open the faulty circuit. Pls consider that if the fault is on the same phase, the protection does not open but the fault is not dangerous for the people.
IT Systems: the stray currents
So far so good but we are missing an important aspect in IT systems: the stray currents.
If the neutral is not grounded, than there is no way back to the source and the person is safe. In reality, unfortunately, even in an ungrounded system the current does find a way back to the source! The issue is that an IT system can extend for kilometres and the larger is the network the larger will be the Parasitic capacitance, or stray capacitance.
Rules of thumbs exist how to estimate the stray currents: here one of them:
- Id is the stray current in A and
- S is the installed power in MVA
For example, if the IT distribution is made of equipment having 100 kVA of installed power, a stray current of 40 mA may flow through the ground resistance of the equipment.
Why are stray currents dangerous in IT systems, because they may create dangerous touch voltages.
Therefore, hereafter is the language used to limit the risk related to stay currents in IT systems.
[IEC 60364-4-41] 411.6.2 Exposed-conductive-parts shall be earthed individually, in groups, or collectively. The following condition shall be fulfilled: In a.c. systems the following condition shall be fulfilled to limit the touch voltage to:
- RA is the sum of the resistance in Ω of the earth electrode and protective conductor for the exposed-conductive-parts;
- Id is the fault current in A of the first fault of negligible impedance between a line conductor and an exposed-conductive-part. The value of Id takes account of leakage currents and the total earthing impedance of the electrical installation.
Bottom line, why is an IT system safe? Because of 4 conditions to be satisfied:
- All exposed conductive parts must be connected together.
- The first fault is detected by, for example, an Insulation Monitoring Device.
- The fault current shall not generate a dangerous touch voltage (≤ 50 Vac).
- A second fault on a different phase must be detected and a short circuit current device must de-energise the devices. Hereafter the language used in the standard:
[IEC 60364-4-41] 411.6.4 After the occurrence of a first fault, conditions for automatic disconnection of supply, in the event of a second fault occurring on a different live conductor, shall be as follows:
a) Where exposed-conductive-parts are interconnected by a protective conductor collectively earthed to the same earthing system, the conditions similar to a TN system apply and the
following conditions shall be fulfilled where the neutral conductor is not distributed in a.c.systems [..]:
- U is the nominal a.c. voltage in V between line conductors (normally 400 Vac in Europe);
- Zs is the impedance in Ω of the fault loop comprising the line conductor and the protective conductor of the circuit;
- Ia is the current in A causing operation of the protective device within the time required in 4188.8.131.52 for TN systems or 4184.108.40.206.
The formula may seem odd, why? If we write it in a different way it will become clearer:
The issue is that Zs is not a normal fault loop current, but the fault loop current of 2 fault loops. The standard is than considering that “each fault loop current” would be half the current going through the full loop. In essence, It’s a way to be conservative!