cables and penetrations

Cables and Penetrations

1.0 Introduction

Normally in any power plant, cabling assumes no significance and it is only required to connect the load to the feeder through an appropriate size of cable. But in a PLANT, because of the importance to Safety and adherence to the relevant standard practices and stipulations, cabling and associated activities assume a lot of significance.

In PLANT following aspects are considered

Segregation Standard recommended practices Cabling – design aspects Cable carriers/trays Fire protection with regard to cables Hermetic cable penetration

Class 1E circuits: the safety classification of circuits those are essential to emergency plant shutdown, containment isolation, plant cooling, and containment and plant heat removal or otherwise essential in preventing a significant release of pollute material to the environment.

1.1 Electrical segregation:

The requirements for the electrical segregation of the cable systems for Class 1E circuits, according to voltage levels, signal levels and vulnerability to electrical noise are as follows.

1.1.1 MV Power cables

Designed to supply power to utilization devices of plant auxiliary systems rated between 601 to 15000 Volts. MV power cables shall be installed with respect to LV cables.

1.1.2 LV Power cables

Designed to supply power to utilization devices of plant auxiliary systems rated at 600 Volts or less.

of 16Prepared By Prasanna Kumar.P.G.

1.1.3 Control cables

Applied at relatively low current levels or used for intermittent operation to change the operating status of a utilization device of the plant auxiliary system.

1.1.4 Instrumentation cables

Used for transmitting variable current or voltage signals (analog) or those used for transmitting coded information (digital). Instrumentation cables shall be installed to minimize unacceptable noise pickup from adjacent circuits and equipments.

1.2 Physical separation and electrical isolation

Physical separation and electrical isolation is provided to maintain the independence of the Class 1E circuits and equipment so that the safety functions required during and following any DBA can be accomplished.

Physical separation is achieved by

Use of safety class structures Separation distance or barriers Combination of both

Electrical isolation is achieved by:

Isolation devices Separation distances Shielding and wiring techniques Combination of any

2.0 Cables and cable trays

Classification of areas where Class 1E and associated circuits are routed.

Non-hazard areas No high energy equipment Limited to C&I functions, power circuits for which are in enclosed trays Control & limiting of introduction of potential hazards the area-possible Limited hazard areas

Plant areas from which potential hazards such as missiles, non-electrically induced fires and pipe failures are excluded.


2.1 Hazard areas

Independence of redundant class 1E systems is maintained by cable routing restrictions, or physical separation or both. Redundant standby generating units, batteries, battery chargers, distribution S/G, are placed in separate safety class structures. Containment electrical penetrations are widely dispersed around the circumference of the containment.

Minimum separation distances are specified to meet the following criteria:

All cables meet fire propagation requirements Non combustible cable trays Fire resistance rating of fire barriers be commensurate with fire hazards Identification of the exposed 1E circuit cable trays is done in a distinct

manner at interval not exceeding 4.5M.

2.2 Shielding and Shield grounding (for MV and instrumentation cables for 1E)

2.2.1 Cable shield: A non-magnetic material applied over the insulation of the conductor or conductors to confine the electrical field of the cable to the insulation of the conductor or conductors. It is required that cables rated above 5 kV shall be shielded, except for special applications or cable designs as per IEEE 690.Shielding can also be used in cables rated less than 5kV to monitor or test cable installation for additional assurance of insulation integrity.

The purpose of an insulation shield is:

To provided a uniform voltage stress over a relatively rough stranded conductor surface

A close bonding between the conductor and the insulation To avoid interspersed voids that may constitute sources of partial

discharge The outer shield being grounded also protects the cable against any

potential that may be induced extraneously.

3.0 Cable carrier system

Cables are normally carried on cable trays and through conduits at a few locations. Cable trays are fixed on the existing walls/ceiling/floors of the buildings. Exclusive walk through tunnels are used where there will be a large number of cables from one building to another and trenches are provided when there are less number of cables. Cable tunnels are advantageous from the point of view of minimum interference to traffic and drainage, good physical protection, ease of addition of cables, shielding effect of the ground mat.


However the disadvantage lies in the high initial cost and the danger that a fire could propagate between cable trays and along the length of the tunnel. The fire hazard is minimized by provision of fire barriers of suitable rating at specified intervals.

3.1 Cable Trays

Cable trays are basically cable carriers supported on cable tray supports. Cable trays may be of ladder type, perforated type, solid type in line with the type of cable to be carried and the importance.

3.1.1 Tray design

Cable tray design is done on the following major requisites:

Required loading and the maximum spacing between the supports. Loading calculations include the static weight of the cables and a concentrated load of 100 Kgs approximately at mid-span.

In case of a ladder type tray the rung spacing is a nominal 9 inches. Design should minimize the possibility of accumulation of fluids and debris

on the covers. Consideration to ventilation and ampacities of the cables is also given.

3.1.2 Tray system design

The vertical spacing, in general is 12 inches, between the trays. Clearances of atleast 9 inches are maintained between the top of the tray and any piping/beam, etc. to facilitate cable laying.

When stacked cable tray configuration is used, a descending voltage level arrangement is followed. And the structural integrity of the components and the pullout values of support anchors and attachments is also verified.

3.1.3 Material of the cable tray

Generally hot dip galvanized cable trays are used in our plants. Wherever the galvanized surface on the steel tray is broken the area is coated to protect against corrosion. However, within the process rooms and corridors the cables are laid in ducts or in pipes.

3.1.4 Grounding

Cable trays are made electrically continuous and solidly grounded and normally a GI ground conductor is run all along the stack of trays and is connected to the grounding grid.


3.1.5 Identification

Cable tray sections are permanently identified with the tray section number as required by drawings/specifications giving reference to the category, group and the area of use.

4.0 Evolution of cable insulation

Natural rubber, which was later vulcanized by adding sulphur and heating and maintaining the temperature to specified levels for cross linking to improve its qualities like moisture ingression, higher tensile strength and to prevent flow under high pressure.

Butyl rubber which is better resistant to strong acids but is sensitive to alkalis. Ethylene propylene Rubber which is synthesized from Ethylene and Propylene. This insulation is not used for higher voltages because of higher dielectric losses.

Silicone rubber which is used for low temperature applications because of its flexibility and for very high temperature applications upto 250 deg Celsius.

Polyethylene which is formed on the polymerization of Ethylene C2H4 is very popular in Electrical insulation application because of:

Low priceProcess abilityResistance to chemicals and moistureFlexibility at low temperatureExcellent electrical propertiesHigh density Polyethylene has enhanced properties of surface hardness, yield strength, heat and chemical resistance.

Cross linking is a process of bonding between the individual polymer structures and causes PE to change from a thermo plastic to a thermosetting plastic.

Cross linking is either done through a chemical process by adding peroxides at specified temperatures or by irradiation.

4.0 Cable installation

Cables are installed in trays with supports that are qualified for the design basis event. During storage the ends of the cables are sealed against moisture and contamination. Other than the generally known guidelines it is also kept in mind


that cable trays filled to the predetermined quantity (design) should not be used for more cables till a proper inspection/analysis is made to ensure that the(1) Ampacities (due to de-rating) does not change from the designed values(2) The actual loadings (physical) do not exceed the design values from the point of view of the seismic requirement.It is also important to ensure that the vertical runs of the cables are secured properly to avoid subjecting the terminals to any excessive tensions.

4.1 Acceptance testing of installed cables To verify that the installed cables are free from any major cable insulation damage during installation I) MV cables are subjected to high potential tests prior to connection to the equipment ii) LV cables are checked for insulation resistance and continuity prior to connection to the equipment and the records maintained.

Documentation is prepared as the work is performed and maintained to furnish evidence of the quality of items and of activities affecting quality.

4.2 Cabling at PLANT

The main cable routes between buildings (with large number of cables) in PLANT are provided through underground cable tunnels and cable routes with small number of cables are through trenches. The cable routes for safety related systems are through underground tunnels only. The cable structures of normal operation are designed to withstand OBE and safety systems for SSE.The following types of cables have been designed for PLANT.

fire retardant cables for normal operation loads fire resistant (fire survival) cables for safety system loads including EPSS

Basic characteristics of Cables for Safety System and Normal Operation systems are given in table -1 &2.

Table-1 Basic Characteristics of Safety System Cables

Location Inside Containment Outside containment

Voltage 6 kV 0.38 kV220 V,110V

6 kV 0.38 kV 220 V

Type of the cable Not

Envisaged

FRHF FRHF LS FRLS FRLS

Class of voltage

1000 V 660 V 7.2 kV 1000 V 660 V

Conductor material

Copper Copper Copper Copper Copper


Conductor insulation

XLPE XLPE XLPE PVC XLPE

Cable inner sheath

Containing no halogen


LS PVC FRLS PVC FRLS PVC

Cable outer sheath



LS-PVC FRLS PVC FRLS PVC

Armour for the cable

Un-armoured

Un-armoured

Un-armoured

Un-armoured

Un-armoured

Oxygen index for the sheath

35 % 35 % 35 % 35 % 35 %

The contents of chlorides (halogens) for sheath

0.05% 0.05% 5% 5% 5%

Fire rating, min

90 90Fire retardant

90 90

Smoke density

10-12 % 10-12 % 60% 60% 60%

HF – Halogen Free;

FR – Fire Resistant (Fire survival);

LS – Low Smoke (fire retardant);

XLPE – Cross-linked polyethylene;

PVC – Polyvinyl chloride .

Table-2 Basic characteristics of Normal operation power and control cables parameters

Location Inside containment Outside containment

Voltage 6 kV 0.38 kV 220 V, 110 V

6 kV 0.38 kV 220 V, 110 V

Type of the cable

HF HF HF LS LS LS

Class of voltage 7.2 kV 1000 V 660 V 7.2 kV 1000 V 660 V

Conductor material

Copper Copper Copper Copper Copper Copper


Conductor insulation

XLPE PVC PVC XLPE PVC PVC

Cable inner sheath

HF HF HF LS PVC LS PVC LS PVC

Cable outer sheath

HF HF HF LS-PVC LS PVC LS PVC

Armour of cableUn-armoured

Un-armoured

Un-armoured

Un-armoured

Un-armoured

Un-armoured

Oxygen index for the sheath

35 % 35 % 35 % 35 % 35 % 35 %

The contents of chlorides (halogens) for sheath

0.05% 0.05% 0.05% 5 % 5 % 5 %

Fire rating, min Fire retardant

Fire retardant

Fire retardant

Fire retardant

Fire retardant

Fire retardant

Smoke density 10-12 % 10-12 % 10-12 % 60 % 60 % 60 %

Routing of the cables from the basic route to the load is made through rigid/ flexible metal pipes.The cables for loads within the reactor containment are routed through sealed penetrations designed for emergency environmental parameters, inside the containment.

The cables cross the internal walls and floors through pipe penetrations and the opening is to be sealed by a fire resistant sealant. Provision of space for accommodating additional cabling, in future, to the extent of 15 % is available. Mutually redundant cables are installed in different routes to prevent simultaneous failure due to a common cause.

4.3 Cable sizing

Having decided the type of the cable the following consideration are made while selecting the cable size.

4.3.1Continuous current rating

Continuous current rating of cables depends largely on ambient temperature and also with the type of the installation and grouping of cables. Proper de-rating factors are considered for the purpose of asserting the actual current carrying capacity of the cable. De-rating factors are applied under the following conditions:


Ambient temperature Ground temperature Type of installation and grouping of cables

4.3.2 Short circuit withstanding capability

For Breaker protected feeder with relays, the minimum cable size depends on the short circuit current of the system as well as on the duration of the short circuit. Due consideration is given while selecting the minimum size of the cable so that it can withstand the specified system fault current for the specified fault duration. At times, the minimum size of the conductor is decided by the short circuit withstanding capacity, especially when feeders are protected with switchgear and relay (as the actuation time of the relay - instantaneous (0.2s) or IDMT (1s) + breaker operating time + a safe margin is taken into account).

4.3.3 Voltage drop

The minimum size of the cable should be such that while carrying the normal running or starting load, the voltage drop at the equipment terminal should not cross the specified limits.

Flexible cables are used for mechanisms of vibration isolators.Special consideration, while designing is also given to the areas where ambient temperatures exceed 40-45 degree Celsius such as reactor containment, near steam valves, equipment enclosures. During an accident condition the atmosphere within a PWR may consist of saturated steam, air, and hydrogen. In addition the equipment could simultaneously be exposed to a typical aqueous spray of Boron, Boric Acid, NaOH solution.

Environmental conditions considered in design of cables are given in table- 3.

Table- 3 Environmental conditions of cables

Power Cabling -areas/locations

Designed ambient temperature during normal operation

Designed ambient temperature under emergency condition *

DG room 43 deg. C 50 deg. C6kV & 0.38 kV switchgear / MCC room 35 deg. C 43 deg. CBattery room 20-25 deg. C 35 deg. CRectifier and inverter room 35 deg. C 40 deg. C


Cable inside containment 40 deg. C 43 deg. C

Selection of materials for use in radiation environments is considered for short and long term effects of radiation. Both normal radiation levels and during accident conditions are taken into account and cables are qualified accordingly.

Ageing simulation is designed, to put a test specimen, in the end –of- life or the plant design life service condition, whichever is earlier and this includes thermal and radiation ageing.

The concept of defense in depth against fire and its consequences includes the use of non-combustible materials. When the use of non-combustible materials is not practical such as in the case of cables, lubricants, etc. their flame retarding properties is considered.

The minimum separation distances for cables/structures are given in table-4.

Table-4 Minimum Distances for Cable Structures

Distance

Minimum dimensions, in mm,in laying

In tunnels, galleries, cable floors and on trestles

In trenches and double floors


Height in air 1800 Not limited, but not more than 1200 mm

Horizontally in air between structures when positioned side-by-side (passage width)

1000 Both side structures not envisaged

Horizontally in air between structures when positioned single-side (passage width)

900 300 with depth down to 0.6 m450 with depth more than 0.6 down to 0.9 m600 with depth more than 0.9 m

Vertically between horizontal structures for power cables having voltageunder 10 kV*

200 150

Vertically between horizontal structures for control cables and communication cables, power cables with section up to 3 x 25 mm2 and voltage up to 1 kV*

100 100

Between support structures (cantilevers) along the length of cable structures*

800 -1000 800 -1000

In air between single power cables with voltage up to 35 kV**

Not less than a cable’s diameter

Not less than a cable’s diameter

Horizontally between control cables and communication cables**

No standard No standard

* Usable length of cantilever must be not more than 500 mm on the straight parts of the route.** The same applies to cables, laid in cable wells.

5.0 Fire protection/Barrier

Fire protection is basically classified into

I) Active fire protection II) Passive fire protection.

As the nomenclature conveys, an active fire protection system is which comes into action in case of a fire, like the water spray or mulsifire system and fights to put down the fire. A passive fire protection system is one which remains as it is even when there is a fire and resists/attenuates propagation of fire through it.


Automatic fire detection devices are installed in areas of high cable concentration. And in PLANT Automatic fire detection and fire fighting system has been provided in the cable tunnels, shafts, etc. After conducting fire hazard analysis, fixed automatic fire-extinguishing systems are provided for high cable concentration areas and spaces below/above false ceilings containing exposed cables, if found required. However, if the activation of the fixed automatic water spray discharge could cause undesirable consequences to sensitive equipment which would negate single failure criteria, such equipments are protected from the spray and sealed against potential water damage due to water traveling along the cable system. If the equipment cannot be protected, an extinguishing system utilizing another extinguishing agent is provided.

In areas where forced ventilation can circulate smoke or a gaseous extinguishing agent, or both, to other areas, mechanical ventilation systems are shut down prior to system actuation and fire dampers are closed by mechanical or electrical releases prior to fire protection system discharge.

Fire barriers or fire stops are passive fire protection systems which prevent/resist/attenuate propagation of fire from one fire zone to the other, maintaining the required fire rating.

5.1 Design objectives of the fire barriers in cabling

To prevent/retard/attenuate flame propagation and spread of fire in cable runs.To prevent delay and minimize the damage due to fire to the cables, preserve the functioning and increase their resistance to fire/heat.To segregate cable runs into compartments with a view to localize a possible cable fire and its spread.To ensure that a single internal fire in one safety group cable system does not expose/affect the other safety group cable system.To ensure that fire in NSR cable system does not affect the SR cable system.

Fire barriers of 3/1.5/0.75 hours rating are provided at

I) the openings in walls and floors where the cables cross from one building to another, one redundant area to another, one floor to another and from one room to another

II) at the entry and exit of cable trenches and along cable tunnels at regular intervals (one in every 30M horizontal – 0.75 hour rating and 20M vertical). Doors at the ends of the cable tunnels are also of 0.75 hour rating.

III) Between groups when the required segregation cannot be adhered to.


The cables in each of the four safety channels of emergency power supply system are installed in separate routes with building/structures having fire resistance of not less than three hours.In case of crossing of cable ducts of different channels of safety systems crossing of cables of normal operation over safety systems cables with a separation distance less than two meters, a coat of a fire resistant compound with a fire resistance of 1.5 hours is provided on the outer surface of the duct . Cables feeding power to the mutually redundant loads of the same channel are also routed separately, with a partition between them having a fire rating of not less than 0.75 hours.

Material of the fire barrier is selected with the objective of meeting the following requirements:

To meet the specified fire rating Should be completely gas tight besides being a effective fire seal Should not contain flammable materials Should be of retrofit design to facilitate addition or removal of cables Should not have any effect on the ampacity of the cables Should remain secure in its place through out the service life and should

not get dislodged during repairs/retrofit operations. Should not undergo shrinkage or cracking after prolonged use. The material should be non-hygroscopic, non-corrosive. Should be compatible with cable sheathing material. Should have a life expectancy of 30/40 years. Should have resistance to chemicals. Should be able to withstand specified radiation dosages.

Testing of the barrier is done by preparing a sample of the barrier with specified number of cables and subjecting it to various tests like impact test, water absorption test, ageing test, vibration conditioning test, fire rating test and hose stream test.

6.0 Cable Penetrations

Hermetically sealed special electrical penetrations are used to pass cables through the Reactor building containment. These Penetrations are leak-tight and thus, the integrity of the containment while crossing of electrical cables through the same is maintained under normal and accident conditions.A typical penetration consists of:

Electrical conductors suitable for connection of cables inside and outside the containment.

Elements ensuring insulation of these conductors.


Elements ensuring leak-tightness, gas tightness, thermal stability and elements of fixation with the containment wall.

Manometer for recording of gas leakage.

6.1General Requirements

Leak-tight cable penetrations are designed in accordance with the containment design requirements. While designing and installing penetrations, effects of postulated accidents and seismic events are taken into consideration.Penetrations are qualified in accordance with the requirements of IEEE-317, IEEE-323 and IEC-60772.The cable penetrations are serviceable under both the normal operation mode and in emergency conditions inside the containment.

6.2 Mechanical Requirements

Electrical penetrations are designed with decontamination capability corresponding to the requirements for the containment. Following are the important features.All the materials in the penetrations are qualified for radiation resistance and do not have to be replaced while in service.

Design pressure: 5.6 bar abs.Design temperature: 150 оСDesign gas leakage through a penetration assembly without open plugs at a differential pressure of 1 bar is 1x10-6 cm3/s. of HeliumAdmissible integrated absorbed dose of a radiant irradiation for life expectancy is 1.57.

6.3 Electrical Requirements

Cable penetrations circuits meet the following conditions of operation.

Insulation of each conductor to withstand high voltage tests in conformity with the applicable standards.

Penetration of thermocouple conductors and connections are designed to take into account credible measurement errors, stipulated by the applicable standards

Coaxial cable penetrations are designed to take into account the transmission requirements

Penetrations are designed to meet the following electrical requirements( as per IEC-60772).

Medium voltage power conductors (above 1kV) to be free of partial discharge (corona) when energized at rated voltage.


The electrical penetrations are adopted to the required properties of the measuring circuits so that the cable penetrations do not deteriorate the measuring circuits

Cable connections and splices of medium & low voltage conductors are capable of carrying rated continuous current before and after continuous rated short circuit currents without causing the connections and splices to exceed the temperature design limit.

The penetrations are designed to withstand stresses resulting from the short circuits without changing leakage characteristics

The electrical integrity of conductors, connections and electrical insulation systems are designed to withstand all electrical environmental loading without failure or loss of function.

Fire resistance of penetrations is similar to that of the containment. Cables penetrations are capable of maintaining the containment integrity

even in case of a cable fire. Penetrations are delivered under dry nitrogen pressure that provides

visual inspection / control of leak-proofness of each penetration after its installation.

Penetrations have a lifetime of not less than 50 years.

6.4 Maintenance requirements

Penetrations are designed to be maintainable. Time required for replacing a damaged feed-through / module or mounting a feed-through / module instead of a plug, will not exceed 8 hours. 6.5 Construction

Electrical penetration assembly consists of modules installed in a steel case and connection boxes mounted on both ends. Modules with electrical wires shall be interchangeable. Penetrations are intended for mounting in horizontal position. The penetration case shall be installed in an embedded pipe, in the reinforced concrete containment wall (1200 mm thick) with steel lining on the reactor side. The penetration is fitted with shrouds, protecting the module's sealing points from mechanical damage. The penetration construction shall ensure the possibility of replacing penetration modules both at the manufacturing plant and under operating conditions.

The penetration keeps its operability under seismic conditions also.

7.0 Figures

1. 6KV cable connection to penetration in the containment

2. 6KV cable connection to penetration outside the containment

3. Connection of two cables to penetration of 16

Prepared By Prasanna Kumar.P.G.

4. 0.38 KV cable connection to penetration

5. Cable carrier lay out

6. 6KV hermetic penetration

7. 0.38KV & 220V AC hermetic penetration


cables and penetrations

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