Mobile generators are indispensable aids when power needs to be supplied to equipment that has no direct access to a permanent installation. This applies e.g. to building and assembly sites, rescue missions, and also to events. Mobile generators can also supply sections of permanent installations in which the failure of the public power supply can result in critical situations, e.g. in buildings used for breeding animals, fire stations. For mobile subsations

Special attention must be paid to protection against electric shock, since the users are typically not very knowledgeable about electricity.

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Unclear Specifications

The range of application possibilities for mobile generators is often matched by the variety of discussions about the implementation of necessary protective measures against electric shock. On the one hand, this is understandable because of the many standards that generators must satisfy, and thus also various statements and assertions regarding the necessary measures. The use of the earth spike in particular is a constant source of misunderstandings. To ensure that the protective devices work properly, the following distinctions must be made in terms of application.

• Independent low-voltage power generation facilities that are not connected to the electricity distribution network.
• Independent low-voltage power generation facilities that intermittently supply permanently installed systems which are not connected to the electricity distribution network.

The most important references for protection against electric shock are the protective measures defined in the basic safety standard DIN VDE 0100-410 (VDE 0100-410):2007-06. These measures are explained in more detail in terms of their application for mobile generators in the new DIN VDE 0100-551 (VDE 0100-551):2017-02 Annex ZC.

In practice, the following basic protective measures are used for mobile generators according to DIN VDE 0100-410 (VDE 0100-410):2007-06:

• Protection by automatic shutdown as described in section 411
• Protective separation with one or more loads as described in section 413.

These are therefore to be considered as two completely separate protective measures which also differ considerably in their practical application.

Protection by Automatic Shutdown

According to DIN VDE 0100-410 (VDE 0100-410):2007-06, automatic shutdown is a protective measure in which

• Basic protection is provided by basic insulation of the active parts;
• Fault protection is provided by protective equipotential bonding via the main earthing bar and automatic shutdown in the event of a fault.

Automatic shutdown in the event of a fault must be assured by a corresponding protective device with isolating properties (e.g. circuit breaker and residual current device RCD). In order to ensure the necessary protective earthing / protective equipotential bonding according to DIN VDE 0100-410 (VDE 0100- 410):2007-06 section 411.3, the exposed conductive parts (of the equipment) must be connected to a protective earth – under the prevailing conditions for each system depending on the earth connection type. Exposed conductive parts that can be touched simultaneously must be connected to the same earthing system individually in groups or together. The earth connection is then divided into one of the following subgroups:

• TN system
• TT system
• IT system

Then, the following necessary measures arise for practical use of mobile generators.

Earthed power supply – TN and TT systems

The star point of the generator must be connected to the earthing system. To do this, adjustments usually have to be made on the generator’s terminal board.

In order to set up the necessary earthing system, an earth spike must be driven in, and the loop or earthing resistance must be determined by a qualified electrician.

One residual current device (RCD) per socket-outlet is required for the TN and TT systems – even with regard to the fact that the generators are deployed outdoors and are designed for use by untrained, unqualified personnel. Accordingly, DIN VDE 0100-410 (VDE 0100-410):2007-06 section 411.3.3 “Additional protection” must also be complied with.

The proper functioning of the protective measure – protection by shutdown – must be tested by a qualified electrician.

Unearthed power supply – IT system

If the generator is operated as an unearthed power supply (IT system), an insulation monitoring device (IMD) conforming to DIN EN 61557-8 (VDE 0413- 8):2015-12 that reports a first insulation fault with a visual and/or audible warning must be installed. If a second insulation fault occurs on another conductor, the system must be shut down accordingly by a circuit breaker. In this case, too, proper functioning must be verified by a qualified electrician. In DIN 6280:1986-10, a loop resistance of max. 1.5 ohms is specified for this.

To summaries, whenever a mobile generator is used as a TN, TT or IT system, an earthing system and a qualified electrician are needed to confirm the proper functioning of the necessary protective and monitoring devices. Of course, this is not feasible in practice and, ultimately, it is associated with costs. To avoid this, the “protective separation” protective measure is used. But here, too, there are a number of things to consider if it is to be usable by individuals without an electrical engineering background.

Protective measure: Protective separation

Protective separation is an independent protective measure in which, in accordance with DIN VDE 0100- 410 (VDE 0100-410):2007-06 section 413,

• Basic protection is provided by basic insulation of the active parts;
• Fault protection is assured by simply disconnecting the electrical circuit from other circuits and earth.

Accordingly, protective separation derives its protective effect from the fact that the secondary circuit is separated galvanically from the mains supply and earth, or, in the case of a mobile generator, the output winding is separated from earth. When an active conductor is touched, the high insulation resistance and low system leakage capacitances (Ze very large) of the secondary circuit to earth means that virtually no current can flow through a human. However, the size of the system leakage capacitance is negatively affected by the length of the wire and the number of loads or the resulting impedance between the active conductor and earth (Z becomes smaller). For this reason, the instruction to limit this protective measure to the supply of an electrical consumable applies generally, and the length of the wire must also be borne in mind. For fault protection, additional measures are still needed:

• The voltage of the circuit with protective separation must not exceed 500 V.
• The active parts of the circuit with protective separation must not be connected to another circuit, to earth or to a protective earth at any point.
• The exposed conductive parts of the circuit with protective separation must not be connected to the protective earth, to the exposed conductive parts of other circuits or to earth.
• Flexible cables and wires must be clearly visible along their entire length in places that are exposed to mechanical loads.

Provided they comply strictly with these requirements, protective separation with a load may also be used by individuals without an electrical engineering back- ground. In practice, however, mobile generators are usually equipped with more than one socket-outlet. Therefore, the requirements of Annex C.3 of DIN VDE 0100-410 (VDE 0100-410):2007-06 apply as well.

Annex C.3 of this standard describes the protective measures with more than one consumable, which may only be operated and monitored by qualified electricians or individuals who have received electrical training.

• C.3.2 In the case of protective separation with more than one consumable, the requirements of section 413 (not including 413.1.2) and the subsequent additional requirements must be met.
• C.3.3 Take precautionary steps to protect separate circuits from damage and insulation faults.
• C.3.4 Connect the exposed conductive parts together by insulated, unearthed protective conductors. These conductors must not be connected to the protective conductors or other exposed conductive parts in other circuits or to any extraneous conductive parts.
• C.3.5 All socket-outlets must have an earth con- tact that is connected to the protective equipotential bonding system and meets the requirements of C.3.4.
• C.3.6 Flexible connecting wires must include a protective earth conductor that functions as a protective bonding conductor and also complies with the requirements of C.3.4.
• C.3.7 If one insulation fault occurs in each of two different items of equipment and various external conductors, shutdown must take place within the time shown in table 41.1 of DIN VDE 0100-410 (VDE 0100-410):2007-06.
• C.3.8 It is recommended that the product of the nominal voltage of the power circuit in volts and the length of the wires/cables in meters should not exceed a value of 100,000 and the total length should be limited to 500 m.

Result

Provided a small number of requirements such as are carefully observed, the protective measure protective separation with insulation monitoring and disconnection represents an effective measure for ensuring protection against electric shock, and also has the advantage that it can be installed immediately if necessary, even without a qualified electrician.

• Use of standard-compliant (undamaged) emergency generators as the current source
• Maximum cable length of 500 m
• High-quality, undamaged conductor material
• Equipotential bonding performed with due care
• Use of operationally reliable, tested consumables (working materials)
• Standard-compliant protective measures

References