Protective resistors RZ

Protective resistors RZ are used for equipment protection in 3-35 kV networks at single arcing ground faults. They also suppress ferroresonance phenomena and associated damages of voltage transformers and other equipment. Resistance neutral grounding allows elimination of accident development at multiple faults and providing conditions for quick and reliable determination of a faulty feeder by relay protection. A protective resistor RZ connected to a neutral circuit can be used in conjunction with surge suppressors for strong limitation of switching overvoltages.

Protective resistors RZ for 3-10 kV networks are manufactured in accordance with Technical Specifications TU 3414-001-73132086-2010.

Protective resistors RZ for 35 kV networks are manufactured in accordance with Technical Specifications TU 3414-002-73132086-2010.


The resistor consists of protective resistor elements (PRE) which manufactured in accordance with Technical Specifications TU 3414-005-11840528-97.

PRE are composed of interconnected heating elements (HE) made of ECOM and placed to the metal enclosure with a dielectric heat-conductive spacer between the enclosure and HE. Hermetic metal enclosure is equipped with a pressure-balancing device inside a PRE. PRE are connected in-series, placed vertically and fixed on the frame. Gap width between PRE is defined by rated voltage of the system where the resistor is mounted and by heat emission in steady-state conditions.

Protective resistor RZ is manufactured from one or more uniform resistive banks.

Uniform resistive bank

Base (1) should be placed on four supports (2). Base insulators (3) are fixed on base (1). Insulation plates (4) fixed on insulators support protective resistor elements (PRE) (5). Insulation plate (7) secures vertical posts (6) rigidly fixed on base insulators (3); this insulation plate is used for fixing PRE on top.

Manufacturing the resistor as vertically oriented single plates provides good heat removal by natural convection from plates to the air. This allows steady-state operation in open-phase mode. According to PUE regulations (Russian Electrical Code), open-phase mode should last no more than 6 hours without disconnection of customers and resistors.

As a resistor consists of single plate elements, it is quite easy to choose proper quantity of elements for providing desired resistance and power in 3-35 kV networks.

Альтернативный текст
Figure – Uniform resistive bank

1 – base
2 – supports
3 – base insulators
4 – insulation plates
5 – protective resistor elements
6 – vertical posts
7 – insulation plates
8 – insulator for system neutral connection

Technical specifications

Rated system voltage
Rated power
Rated resistor current
Operating time of RZ at single line-to-ground faultsPermissible surface heating temperature of PRE
Life time
3, 6, 10, 35up to 200from 1 to 2000continuous110not less than 30 years

Construction of a resistor should provide easy maintainability.

Resistors RZ do not include parts and materials that are dangerous for people’s life and health and for the environment.

The manufacturer guarantees that the protective resistor RZ conforms to requirements of technical documentation if the consumer operates and maintains the equipment as per provided instructions within 24 months from the date of delivery of the resistor to the consumer.


Resistors RZ are placed on the open area of outdoor/indoor switchgears. Connecting a resistor to the neutral should be done through a disconnect switch for the purpose of isolating a resistor for its maintenance and inspection.

In accordance with PUE (Russian Electrical Code), the resistor RZ should have a cage 2 meters high (or higher), the distance between structural elements of the resistor and elements of the cage should conform to PUE regulations (Russian Electrical Code).

The resistor may not be enclosed according to PUE (Russian Electrical Code), when installing it on the foundation supports providing the porcelain insulator height above ground structures to be not less than 2.5 m.

The resistor is equipped with a special insulator for system neutral connection.

Connecting the resistor RZ to the neutral of 6-35 kV networks can be done through a cable, a wire or a bus bar which can be connected to the bottom (or top) insulator depending of the type of the connection.

The type of the connection is determined by the distance between the disconnect switch and the top flange of the resistor, or other features of the substation.

When using a cable, it is connected to the bottom insulator placed on the basement. When using a bus bar or a bare wire passing under the cage, they are connected to the top insulator placed on the vertical post. The top insulator increases overall height of the resistor.

A terminal of the last PRE is connected to the grounding bolt through a bus bar. “Insulated terminal" can be performed on request. It means that the bus bar is connected to the separate insulator placed on the basement.

Grounding of the bank of resistive elements is done by connecting a grounding conductor of a ground loop to grounding bolts of the bank.

It is allowed to mount the resistor on other supports, e.g. beams or frames, made by welding of channel bars, that provides steady state position of the resistor. Supports should carry 1.5 times the weight of the resistor.

Resistors with required rated resistance and dimensions can be manufactured on request.

Connection diagram

The resistor is recommended to be connected to the neutral of the transformer with “ungrounded wye-delta” winding connection.

In the absence of a brought out neutral terminal, resistors are connected to the neutral of a FMZO zero-sequence coupling filter through a disconnect switch.

If a resistor and a ground-fault neutralizer are used, they are connected to the neutral of the transformer independently.

Designation of the protective resistor RZ

RZ – A – B – C – D

RZ - type of a protective resistor

A - rated resistance, Ohm

B - rated power, kW

C - rated voltage, kV

D - climatic modification, arrangement category in accordance with GOST 15150

Example of the request when ordering

Protective resistor RZ for neutral grounding with a resistance 300 ± 10 % Ohm, rated power 40 kW, to be installed in the system with rated voltage 6 kV, having climatic modification UHL, and 1st arrangement category in accordance with GOST 15150.

RZ – 300 – 40 – 6 – UHL1


UHL1 means:

UHL – operation in cold temperate climate;

1 – outdoor installation.

Since 1998 LLC BOLID have manufactured and put into operation about 1000 protective resistors RZ for the rated voltage 6-35 kV

  • Schemes of power distribution [hydro-electric power plant, CHP (Combined Heat and Power)]
  • Auxiliary networks of power plants
  • Various distribution substations
  • Industrial power supply networks

Protective resistors allow reducing of equipment failures 4-6 times and decreasing of undersupply of energy to customers.