The ACER VOLTAGE Ltd. company was founded in 1992 in Hradec Kralove, originally called ACER. It is a purely Czech company which is engaged in the development and production of surge arresters and overvoltage limiters for the  protection of LV and HV power distribution networks and protect equipment against lightning and switching overvoltage. Furthemore, it also focuses on the production of composite insulators.
Products of ACER VOLTAGE company protects of its customers‘ equipment from the devastating effects of the surge.

This company acted since 2006 under the name ACER HK Ltd. and in the end 2014 moved into a new place of work, where it remains. In 2016, the company renamed to its current name ACER VOLTAGE Ltd. that better reflects its focus and activities.

In the past, the company developed and manufactured varistors based on ZnO surge arresters in the LV applications and then another surge arresters made from them. The first type was a surge arrester SP 0.440 / 10, which has been improved and is now produced only in design SPB * / 10 in various voltage levels acc. to type of installation. Production of ZnO varistors was abolished in 2006, and for current and surge arresters are only used varistors from external suppliers. Another significant part of production program are polymeric arresters for HV networks of 1-39 kV for AC or DC networks . They consist of the entire model range, which is constantly expanding. In addition, it is possible to produce nonstandard types according to customer requirements and specifications. Details about today‘s sortiment ACER VOLTAGE can be found at our catalog or website. Recently was expanded assortment of products with gas-filled power arresters.

The Company has acquired and regularly renews certification according to DIN EN ISO 9001 certified.

ACER VOLTAGE is a leading company supplying technology for power and automation that enable energy and industry could increase its efficiency while reducing their impact on the environment. ACER VOLTAGE products are delivered to 42 countries and are always close to its customers. With superior technology, global scale, deep industry expertise and local knowledge, we can offer our customers products, system solutions and services that help improve the reliability of their transmission networks.

Due to focus and strengths in technologies for power and automation, we strive for organic growth. Our global manufacturing base ensures consistent products and systems of the highest quality produced in the ACER VOLTAGE for customers worldwide. Our customers have easy access to a full range of products, either directly with us as the manufacturer, or through a network of distributors or wholesalers, which is constantly expanding.To ensure the quality of products the company ACER VOLTAGE performs 100% inspection at production testing equipment. With these devices are controlled precise technical specifications throughout the production company ACER VOLTAGE.

Definition of overvoltage

Overvoltage is voltage that exceeds the maximum value of operating voltage in an electric circuit.

Pulse overvoltage, its formation and division

Pulse overvoltage is short-term overvoltage, lasting in the order of nanoseconds up to milliseconds. It is one of the most noticeable and most harmful manifestations of electromagnetic interference (perturbing influences) and it poses a threat especially to electronic equipment containing semiconductor components.

According to its origin, pulse overvoltage is classified into:

  • atmospheric overvoltage (LEMP – Lighting ElektroMagnetic Pulse)
  • switching overvoltage (SEMP – Switching ElektroMagnetic Pulse)
  • overvoltage formed during discharges of static electricity (ESD – ElektroStatic Discharge)
  • overvoltage due to nuclear explosions (NEMP–Nuclear ElektroMagnetic Pulse)

Atmospheric overvoltage (LEMP)

They are the most dangerous and is induced primarily by thunderstorms with lightning discharges. Overvoltage may occur between a phase and the earth, or between phase conductors. It is caused primarily by thunderstorm activity, specifically by lightning discharges. Overvoltage due to lightning manifests itself most on overhead lines and in sections of unshielded cables.

Atmospheric overvoltage may be generated by:

  • a direct or near strike of lightning into a lightning conductor, metal structure, cable...
  • the destructive effect of lightning current is given by high energy liberated in a short time, causing:
    • a voltage drop on earth resistance
    • induced voltage in loops
  • a distant strike of lightning into overhead lines, causing surges of overvoltage following even after a cloud-cloud discharge or after a lightning strike near the line
  • a distant lightning strike diverted to the grand causing a lightning-channel field

Switching overvoltage (SEMP)

They are very frequent overvoltage that occurs in both low-voltage and high-voltage networks.

Switching overvoltage is generated by industrial activities:
  • when great loads, especially inductive ones, are switched on and off, e.g. transformers or electric motors or even small household appliances, e.g. refrigerators, freezers
  • in the event of short circuits in a distribution network and the like.
Invisible voltage pulses that are immeasurable by usual means last only several millionths or thousandths of a second, yet they can cause the destruction mainly of electronic equipment, sometimes even a short circuit and subsequent fire.

ZnO Overvoltage Limiters

Thanks to the use of voltage dependent resistors, mostly zinc oxide ZnO, the limiters are capable of limiting a follow-on current after overvoltage has died away. This property allows to use such ZnO resistors as surge arresters without ignition spark gaps.

The parameters on the basis of which we design the limiters are as follows:

  • Continuous operating voltage of the limiter UC  – it represents the maximum value of voltage connected permanently to limiter terminals at mains frequency.
  • Rated voltage of the limiter UR – it represents the maximum effective value of voltage for which the limiter is designed while the correct function under conditions of temporary overvoltage at mains frequency is maintained. Such voltage is defined as voltage to which the limiter is exposed for 10 seconds following previous stress.
  • Protective level of the limiter UP – is voltage on terminals at a given shape and peak value of current passing though.
  • Nominal discharge current IN – the peak value of an atmospheric current pulse that is used for the classification of overvoltage limiters.
  • Residual voltage URES – it represents residual voltage on the overvoltage limiter. It is actually the peak value of voltage that appears between terminals of the overvoltage limiter when discharge current is passing through it.
  • Working temperature ϑ – it represents the range of permissible ambient temperatures stated by the manufacturer for the limiter to work properly.
  • Line discharge class – a number expressing the ability of the overvoltage limiter to absorb energy in the event that long lines are discharged.

Overvoltage resistance of ZnO limiters

By the magnitude of permissible energy, the overvoltage limiters are divided into five classes. The higher the class, the higher is the energy capacity of the limiter. The energy that the limiter has to absorb during overvoltage rises with the voltage of the network in which it is used. Voltage rises slower than energy. Limiters in networks with higher voltage must have a greater energy capacity than limiters in networks with lower voltage. The selection of the energy class and of the rated discharge current is based on the frequency at which the energy capacity is exceeded in the given application.
An energy class expresses the ability of the limiter to absorb both atmospheric and switching overvoltage. It is stated in the units kJ/KV of the limiter’s voltage and is independent of nominal voltage.
Energy classes 1 to 5 divide the limiters into groups according to the magnitude of the permissible energy of overvoltage that they are able to absorb without being degraded or without losing heat stability at operating voltage. The higher the class, the greater is the energy capacity of the limiter.

The energy classes and an example of their use

  •  Class I.   – use in HV networks without classification of class (5kA) or class 1 or 2 (10 kA)
  •  Class II.  – use in 110 kV networks
  •  Class III. – use for 110-400 kV networks and for cable networks
  •  Class IV. – 400 kV long lines
  •  Class V.  – extremely extensive 750 kV cable networks

Sizing of the overvoltage limiters

The overvoltage limiters are sized based on a particular position in the network, i.e. whether they are to protect e.g. a line outlet, a line transition into a cable or transformer. Whatever their position, they have specific conditions for protection and overvoltage stress.

Selecting the operating voltage UC of limiters

The operating voltage, voltage-current characteristics and all voltage parameters of the limiter are dependent upon the height of column of blocks. By contrast, all voltage parameters are set by the selection of operating voltage UC. Incorrect selection of voltage UC may have a considerably negative effect on the limiter’s function.

Incorrect selection of voltage UC may have a negative effect on its function:
If a low UC is selected, the protective level URES and also the risk of failure of the protected device associated with it will be favourably low. On the other hand, however, there will be a risk of thermal stress on the limiters caused by temporary overvoltage, so the probability of them failing will be high.
If a high UC is selected, the risk of failure of the limiters owing to temporary overvoltage will be insignificant but a high protective level URES will imply a higher probability of the protected devices being destroyed.
The correct selection of continuous voltage UC of the limiters should mean optimum parameters of protection, hence a balanced risk of reliability of the supply for both causes.
The protection parameters can be improved by connecting the limiters as close to the protected device as possible with as short interconnecting wires as possible!
The limiters limit voltage to a limiter’s protective level UP. A limiter’s protective level UP is voltage on terminals with a given shape and peak value of current passing through. Values characterising a limiter’s protective level can be found in our catalogue. It is a limiter’s residual voltage URES.

Characteristics of overvoltage protection of LV and HV networks

In LV and HV distribution networks with overhead lines, it is necessary to protect equipment primarily from atmospheric overvoltage. Switching overvoltage reaches substantially lower current and voltage levels than atmospheric.

The greatest overvoltage in cable networks without connected overhead lines is caused by short circuits and/or switching.

The primary task of protective measures which are economically fully justified is to protect the equipment of LV networks from destruction by atmospheric overvoltage by installing surge arresters and, at the same time, to enable the protection of installation by adequately reducing overvoltage in the network.

Principles for positioning and connecting in LV networks

The overvoltage limiters in TN-C networks shall be connected between a phase conductor and a PEN conductor (in star) at the place of its earthing.
In the event that the overvoltage limiters are positioned at a place where there is no earthed PEN conductor, the earthing shall be carried out through a separate earth electrode. A 1-metre earthing rod or another equivalent earth electrode is considered to be sufficient. The magnitude of resistance of the earthing of the overvoltage limiters is not decisive for their functioning. When designing and earthing, the procedure as per PNE 33 0000‑1 shall be followed.
In absolutely exceptional and justified cases, overvoltage limiters connected between a phase conductor and PEN conductor are not to be earthed.
The overvoltage limiters in TT networks shall be connected between line conductors and the main protective PE conductor, from which discharge current is diverted to the ground via a test clip, earthing wire and earth electrode.

Generally applicable rules for connecting HV limiters

Four rules, which can be applied generally to protection in VN networks, follow the above characteristics:

  1. The overvoltage limiters and the device that is to be protected must be earthed to a common earthing system. The galvanic interconnection between the earthing terminals of the limiters and the earthing of the protected device must be as short as possible.
  2. The total length of conductors a and b of connection of the limiters to the protected device must be as short as possible.
  3. It is always recommended that conductor b should be as short as possible or at least shorter than conductor a.

  4. Strip conductors are more suitable for connection than those with circular cross-section as with the same cross-section strip conductors have smaller inductance and pulse losses of overvoltage in them are smaller. The minimum size of a connecting conductor is 6 mm. The minimum width of a strip conductor is 12 mm.

    Above all, the installation of overvoltage limiters is prevention of possible damage. A seemingly considerable cost of such protections tend to be only a fraction of a per cent of the acquisition value of the technology protected and a negligible sum for possible damage caused by breakdowns and destruction of technological equipment. Unprotected electric distribution systems, computer and data networks always pose a considerable risk to their users. 
 One-line diagram of protection with the marking of sections of conductor a and conductor b 
 Connection of the limiters to the transformer 
 Ground plan of the earthing network of limiters at the transformer 

 Connection of the limiter: (a) correct connection, (b) incorrect connection 


Maintenance and inspection of overvoltage limiters

Complexly recommendations for maintenance and inspection of overvoltage limiters ZnO are specified in EN 62305-4, where such control is divided into visual inspection.alternatively complete revision include important electrical measurements. Especially this control is recommended to make:

- After any changes in protective elements belonging to the system installation
- Periodically, at least 1 year
- After direct strike to the protected instalation