Class II Based Signalling Power SuppliesTahir Ayub

1

Senior Design Authority EngineerIP Signalling, Signalling Innovations Group

Date 18.08.10 Tahir Ayub Railway LV Systems 2

Signalling Power SystemsUtility Intakes

Signalling Supply Point (SSP), Principal Supply Point (PSP)

Signalling Power Distribution System

Interface to final Signalling Loads

Signalling Distribution System

Utility PSP FSP LOC Asset

IT System Operation

PSP/SSP

Line side Equipment

650/110 Tx

Signaling Load

Power Enclosure

PSP 400/650 TX

Earthing System (RA)

325V 325V

Ce Ce Ce Ce Ce Ce

650V

IT Individual – First Fault (ECP)

Line side Equipment

650/110 Tx

Signaling Load

Power Enclosure

Ce Ce Ce Ce Ce Ce

650V

IF

IF

IFIFIFIFIF

IF IFIF

IF IF IF

RF

Earthing System (RA)

PSP 400/650 TX

Elevated Voltage between exposed conductive part and earth dependant on cable length and impedance of Ra. Very unlikely to exceed 60V.

IT Individual – First Fault (Cable)

Line side Equipment

650/110 Tx

Signaling Load

Power Enclosure

Ce Ce Ce Ce Ce Ce

650V

IF

IF

IFIF

IF IFIF

IF IF IF

RF

Earthing System (RA)

PSP 400/650 TX

Residual risk of Direct Contact on exposed cables. The risk is consistent with installations with unarmoured cablesIF Very Small

IT Individual – Second Fault

PSP 400/650 TX

IF

IF

IF

IF

RF

RF

Residual risk of Direct Contact on exposed cables. The risk is consistent with installations with unarmoured cables

IF IF IF

IT Individual – Second Fault (ECP)

PSP 400/650 TX

IF

IF

IF

IF

RF

RF

Elevated Voltage between Exposed Conductive Part and Earth likely to be very small (<60V)

IF

IF IF IF

Vt Vt

Vt Touch VoltageIF Fault Current

IT Individual–Second Fault (Same ECP)

PSP 400/650 TX

IF

IF

IF

IF

RF

RF

Elevated Voltage between exposed conductive parts and earth likely to be > 60V

IF

IF IF IF

Vt

Vt Touch VoltageIF Fault Current

IFIF

System protection not rated to disconnect on L-L or 2 L-E fault

IT Individual – Line to Line Fault

PSP 400/650 TX

IF

IF

IF

IF

RFIF

IF IF IF

Vt Touch VoltageIF Fault Current

IFIF

Protection must operate before thermal damage

IT Collective – System

650/110 Tx

Signaling Load

Power Enclosure

Earthing System Typically 10 Ohms (SSI8503)

RF

Line side Equipment X

650/110 Tx

Signaling Load

Power Enclosure

Earthing System Typically 10 Ohms (SSI8503)

Other LOC or REB Installations

Circuit Protective Conductor (CPC)Collective earth

CPC

Line side Equipment 1

PSP Earthing System

PSP 400/650 TX

IT Collective – First Fault (Cable)

PSP 400/650 TX

IF

IF

IF

IF

RF

Elevated Voltage between Exposed Conductive Part and Earth likely to be very small (<60V). All location cases will see an elevated voltage. Vt dependant on local earth impedance and cable distribution length

IF

IF IF

Vt Vt

Vt Touch VoltageIF Fault Current

CPC

VtVt

IT Collective – Second Fault

PSP 400/650 TX

IF

IF

IF

IF

RF

Elevated Voltage between Exposed Conductive Part and Earth likely to be very lethal (>60V) and is dependant on voltage divider produced by current passing through the CPC and Local earth. This could reach half system voltage depending on all location cases. Vt dependant on local earth impedance and cable distribution length

IF

IF IF

Vt Vt

Vt Touch VoltageIF Fault Current

CPC

VtVt

RF

Protection must operate to manage lethal touch voltages

IF

IT Collective–Second Fault (CPC Optimisation)

PSP 400/650 TX

IF

IF

IF

IF

RF

Elevated Voltage between Exposed Conductive Part and Earth likely to be very lethal (>60V) and is dependant on voltage divider produced by current passing through the CPC and Local earth. This could reach half system voltage depending on all location cases. Vt dependant on local earth impedance and cable distribution length. If Local earth is not maintained then the elevated voltage could be lethal at the ECP.

IF

IF IF

Vt Vt

Vt Touch VoltageIF Fault Current

CPC

VtVt

RF

Protection must operate to manage lethal touch voltages

IF

Local Earth Impedance utilised to optimise CPC

IT Collective – Line to Line Fault

PSP 400/650 TX

IF

IF

IF

IF

IF

IF IF

Vt Touch VoltageIF Fault Current

CPC

RF

Protection must operate before thermal damage

Class II Engineering Strategy

Engineering Strategy Reduced CAPEX costs without reducing safety Improvement in Personnel Safety Reduction in System Risk Compliant to Standards Reduced Maintenance Burden Greater Tolerability to DC Corrosion Simplified Earthing and Bonding in AC Electrified Areas Ability to integrate into Legacy Systems

Class II Proposal

Reduce the probability of insulation failure in the LOCs and REBs to a negligible level so that power supply equipment enclosures need not be earthed for protection, so earthing or bonding to a CPC is not necessary.

Utilising Class II Equipment as a protective measure against electric shock

Utilising a 2-core cable Distribution Strategy in place of a 3-core Strategy

Realise the opportunity of complying with the requirements of BS 7671

Realise the opportunity of reducing costs associated with signalling power supplies

Class II Strategy

No Circuit Protective Conductor (CPC)

Class II Strategy

Double Insulation or Reinforced Insulation

Class II ScopeThe Class II Based Design Solution is to be implemented into:

New Build Signalling Power Distribution Systems

Existing Signalling Power Distribution Systems [3-core Armoured cable - CPC based system]

Legacy Signalling Power Distribution Systems [2-core Unarmoured cable system]

Design Strategy

Location Case/REB Equipment

Class II Switchgear Housings

Class II Signalling Transformers

Encased Cabling between Class II Switchgear and Class II Signalling Transformer

Cable Consumables (Glands, Joints, Terminations etc)

Distribution Equipment 2 Core Enhanced Unarmoured Cable

OR 2 Core Armoured Solid Bonded Cable

Solid Bonded Armoured Cable SolutionE

PR

PSP 400/650 TX

Line side Equipment X

Class II 650/110 TX

Signaling Load

Class II Power

Enclosure

IM Line side Equipment 1

Class II 650/110 TX

Signaling Load

Class II Power

Enclosure

Earthing System (RA) Earthing System (RA)

BS 5467 Armoured Cable

MET MET

Gapped

Enhanced Unarmoured Cable SolutionE

PR

PSP 400/650 TX

Line side Equipment X

Class II 650/110 TX

Signaling Load

Class II Power

Enclosure

IM Line side Equipment 1

Class II 650/110 TX

Signaling Load

Class II Power

Enclosure

Earthing System (RA) Earthing System (RA)

MET MET

Enhanced Un-armoured Cable

Risk Assessment Results Risk of electrocution inside the LOC will, in practical terms, be removed by the

introduction of Class II equipment.

Power cable options ranked in order of safety are as follows:

1.Armoured, solid bonded (=1st)

2.Enhanced unarmoured (=1st)

3.Armoured, resistance bonded

4.Armoured, direct bonded

Other performance factors considered:

• Reliability, Availability, Maintainability,

• Logistics and Whole-Life Costs.

ConclusionThe Enhanced Unarmoured Cable option presents the highest overall level of Safety.

18 February 2014Document Title25

Legacy, Existing and Class II Schemes

Feature Legacy Existing Class II

Safety < Norm >Availability of supply = Norm >

Compliance with Standards N Y Y

Maintenance burden > Norm* <

Ease of fault location = Norm =

Resilience to corrosion > Norm >

Immunity to stray a.c. currents > Norm >

* Assumes design relies on earth electrodes as well as CPC to control touch voltages

Stage 1 Basic Comparison Table

Maintenance Comparisons with Legacy and Existing Systems

Maintenance factor

Solid Bonded Armoured Cable(Class II)

Enhanced Unarmoured Cable(Class II)

1st inspection Reduced dependency on LOC electrode condition No dependency on LOC electrode condition

Periodic inspection

Reduced dependency on LOC electrode condition

Armour continuity/loop impedance test requiredNo dependency on LOC electrode condition

Mid life degradation

Reduced dependency on LOC electrode condition

Armour/gland corrosion

No dependency on LOC electrode conditionCable troughs and ducts maintained in good order

Impact of possession restriction

No impact No impact

PRAMS Conclusion

Cable options

ranked in descending

order of preference

PRAMS ranking of cable options0 = lower, 1 = higher

Performance Reliability/Availability Maintainability

Safety

Personnel System

Enh UnArm

1 1 1 1 1

Arm solid 0 0 0 0 0

Cable Impact & Crush Tests

Impact Tests Crush Tests

Cable Damage Tests

Impact Tests• Armoured Cables maximum 6 impacts to failure• Enhanced Unarmoured Cables 26 impacts, no failure

Crushing tests• Armoured Cables maximum 1.2 tonnes to failure• Enhanced Unarmoured Cables, minimum 2.2 tonnes to failure

The tests showed that the Enhanced Unarmoured Cable design has greater resistance to impact and crushing faults than a standard Armoured cable.

Safety Case

The update of the Safety Case from Stage 1 was undertaken to incorporate the results of the FMECA and PRAMS analysis

The report presents evidence that the project has reached a stage where the identified design options, may be selected for detailed design.

Conclusions

• There is no identified safety reason to prevent the design development stage of Class II Equipment using Enhanced Unarmoured Cable proceeding.

• It is important to recognise that Armoured Solid Bonded Cables remain a viable alternative both in terms of risk and standards compliance however, FMECA analysis identified the highest failure mode criticality is associated with the use of Armoured Cable.

New Standards

NR/L2/ELP/27408 Product Specification for Signalling Power Distribution Cables

NR/L2/ELP/27409 Product Specification for Functional Supply Points (FSP)

NR/L2/SIG/30007 [Issue: 2] Product Specification for Power Transformers for Signalling Systems

NR/L2/ELP/27410 Specification for Class II Based Signalling Power Distribution Systems

Date 08.11.12 32

System Transformation

PSP Installation

Signalling Earthing System

OvercurrentDevice

Enhanced Unarmoured Cable

Insulation Monitor

Class II FSP Switchgear Assembly

Conduit System

Class II Hybrid FSP Isolating Transformer

PSP Earthing System

Enhanced Unarmoured Cable

Signalling Earthing System

Enhanced Unarmoured Cable

IsolatingTX

Signalling Load Signalling Load

Date 08.11.12 33

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