Earth leakage detection & risk mitigation

B. Sambi Reddy [Rtd. CSTE],Chief Engineer, Efftronics Systems PVT. LTD

November 19, 2019

1 Introduction

• Usage of copper cable for transmission of failsafeinformation through electrical supply for control-ling of signalling elements and getting their sta-tus is the current practice on IR.

• Copper medium is prone to electro-magnetic in-fluences. Even though certain precautions aretaken to mitigate the risks still a lot has to bedone.

• This article discusses the methods to detectearth leakage failures making use of ELD anddata logger

2 Risks of Earth Faults - CaseStudies

Earth faults influences signalling system in twoways -

1. Makes the system vulnerable to external surgesreducing availability of the system

2. Can cause unsafe side failures of signalling sys-tem Impact of lightning on signalling systemwith cables with earth fault

Impact of lightning on signalling systemwith cables with earth fault

• Striking of lightning a few kilometres away fromthe location of earth fault on signalling cable canintroduce a surge into the power supply systemthrough galvanic coupling as shown in figure 1.

• This can cause damage to power supplysystem and all the signalling elements thatare connected to the power supply at thattime.

Case 1 -Secunderabad RRI -1998 as shown infig. 9

• Intermediate termination of distant cable is doneat advanced starter location box.

• Conductors of distant signal were earthed at theentry of location box due to damage of cable in-sulation. Tail cables of advanced starter werealso damaged at the same location.

• This caused galvanic coupling of power supplyin both the conductors and the green lamp ofadvanced starter signal was lit without operationof signal.

Case 2 -Kazipet RRI-1998 as shown in fig. 3

• Tail cables carrying point detection supply weredamaged after packing with crowbars by trackmaintenance staff at Kazipet RRI on SCR. Itcausing earth fault on all the conductors.

• Point was in normal when fault occurred. Sub-sequently, command for setting point to reversewas generated in a route setting operation.

• During operation commenced by unlocking ofpoint. As soon as the point machine was un-locked, detection relay [NWKR] dropped. 24 VDC supply was already available on the RWKRtail cables RWKR picked up bypassing the de-tection contacts of point machine and cut offpower to point machine.

• Point was in normal with unlocked point ma-chine and reverse detection was available. Signalwas cleared to an unintended line.

3 Insufficiency of design pre-cautions Floating supplydouble cutting & cross pro-tection

1. Railway signalling circuits are designed to workon floating supply circuit to providegalvanic iso-

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Figure 1: Galvanic coupling between lightning and earth fault introducing surge

Figure 2: Earth leakage causing lighting of unintended green lamp of advanced starter

lation with other supplies with earth return.

2. Double cutting and cross protection are providedto prevent the interference of spurious suppliesin case of earth faults.

3. Cross protection becomes ineffective if the short/ earth fault occurs say at a distance of 1 kmfrom cross-protection as the current drawn bythe short wont blow the fuse of interfering source.

4. All the above safety features become ineffective

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Figure 3: Out of correspondence of point due to earth fault of detection cable

– if there is no system of detecting firstearth fault and rectifying it before sec-ond fault occurrence

– in case multiple faults occur with oneincidence of interference

4 Leakage detection

1. Simple method to measure leakage Leakage de-tection by periodical testingwas one of the processesfollowed. One of the popular methods of testing isshown below.

If the positive busbar or the conductors carryingpositive supply has leakage lamp is lit when the bulbis connected to negative busbar and earth. It is viceversa, in case of fault in negative busbar conductor.

This method is crude as the lamp brightness is theonly way to find severity of leakage which dependson the personal assessment of the staff rather thanon data.

2. System followed on British railways[where online ELDs are not provided]

On IR, process for attending leakage fault is notelaborated. Process followed by British Railways isgiven below which may be of use to develop processby IR.

i.Manual measurement of leakage

(a) Two voltage measurements are made volt-age of each conductor with respect to earthsay V1 & V2 shown in the diagrams below.

(b) For DC busbar, measurements are madedirectly. For AC busbar, measurement ismade through an adopter shown below.

ii. Reportable acceptable safety ranges ofleakage values

DC Busbar individual values of V1&V2 and sumof V1&V2 [irrespective of polarity] represents mea-sure of leakage

iii. Attending leakage failure - severity ofleakage - roles played by staff

a. Classification of severity of leakage & action tobe taken

• The value of leakage for classification of sever-ity of leakage is fixed irrespective of number offunctions fed or length of cable fed by the busbar.

• The safety value for leakage can be common forall lengths of busbars. But it is desirable to

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Figure 4: Double cutting and cross protection

Observation Action1 Less than reportable

range - no deterrioration- compared to previousmeasured value

No action

2 Less than reportablerange - trend is worseningsignificantly from previousresults

Informsupervisor

3 Less than reportablerange - but change is fast- compared to previousmeasured value

Informsupervisor

4 More than the maxi-mum acceptable value

Informsupervisor-within 24hours

5 More than the maxi-mum safety value

Taken action

Table 1:

keepthe values of reportable & acceptable ac-tions based on the length of cable / number offunctions fed by the busbar

b. Responsibilities of various levels of main-tenance personnel

Role Responsibility1 Responsible

- Person whoperform theactivity

Responsible for action &implementation

2 Accountable-Person whois ultimatelyaccountable

Has powers to say YES/NO for the action to betaken

3 ConsultedPersonsto be con-sulted beforedecision

Helps in decision takingfor action [two-way com-munication]

4 InformedPersons tobe informed

Persons to be informed af-ter a decision is taken oraction is taken [one-waycommunication]

Table 2:

5 ELD Earth Leakage Detec-tor to monitor and measureearth leakage

ELD by continuously monitoring the leakage pro-vides alarm when it exceeds set limit.

1. ELDs as per RDSO spec 256 / 2002 can detect

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Figure 5: Testing of leakage in negative conductor bulb connected to positive conductor and earth is litthrough path 1

Figure 6:

leakage from 2 K ohms to one Mega ohm

2. ELD capacity is mentioned in terms of channels

each supply is monitored by one channel.ELDmonitors leakage in 110 / 60 / 24 / 12 V DC &

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Figure 7:

Figure 8:

AC supplies.

3. ELD provides audio-visual alarm as soon as itdetects the leakage beyond set limits. Mainte-nance staff available at ELD has to take actionto identify the fault and rectify.

4. ELD has a potential free contact which operatesfor the first leakage failure. Potential free contactis wired to data logger as digital input to sendonline SMS to maintenance staff.

6 Limitations of present ELD

1. ELD can only find leakage but cannot identifythe defective conductor

2. ELD works on AC supply only it is difficult toensure uninterrupted AC supply. Because of thisnumber of false warnings are generated when-ever power is interrupted to ELD. Option forworking on 24 V DC to be provided

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Figure 9: ELD detecting earth leakage

3. ELD cannot detect second leakage fault even af-ter first fault disappears unless ELD is reset.Real benefit of monitoring ELD by data loggeris not transferred to the user because of this lim-itation.Some railways [WCR] conducted trialson auto-reset type ELD successfully. ELD withauto-reset feature is being deployed on EDFCproject successfully.

4. At present ELD is providing only potential freecontact as interface to data logger. There is needto provide the actual value of leakage through aport to data logger. From the value obtained, itis possible to decide whether it requires immedi-ate action or not. It paves the way for predictivemaintenance.

5. While the first limitation can be taken care ofby data logger; remaining can be addressed bymodifying the RDSO spec.

6. The following diagram presents the events in caseof manual reset & auto reset of ELDs

7 Usage of Data logger to iden-tify defective conductor alongwith ELD

1. Most of the insulation failures occur due to dam-age of cable conductors connected to a signallingelement.

2. Since a busbarpowers number of signalling ele-ments, unless the element is identified, it is dif-

ficult to rectify the fault.

3. Data logger through the relays monitors the sta-tus of signalling elements. It also monitors theleakage fault occurrence.

4. By developing a suitable software, it is possibleto precisely identify the signalling conductor re-sponsible for the leakage.

5. However, if leakage occurs in the common por-tion of bus-bar, it cannot be identified

From the state of change of TPR, NWKPR,CHYR, NWR, HR & LXCR it is possible to findout the time of supply withdrawal from the cableand application of supply to the cable conductors.From ELD contacts it is possible to find out the faultappearance and disappearance [in case of modifiedELDs]. When both the events are linked, defectiveconductor can be declared on real time basis.

8 Identification of defectiveconductor with ELD & datalogger field results

Software solution through data logger to identify de-fective conductor is provided on NCR with old ver-sion ELD [with manual reset] and on EDFC withnew version ELD [with auto reset].

The typical alarms generated during testing onAgra division with manual reset ELDare given be-low:

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Figure 10: ELD with manual reset fails to detect 2nd earth fault

Alarm 1 & 3 are generated by simulating earthfault on 01 CTPR and 9 HECR conductors. Alarm 2is generated when the fault was simulated on busbar.

The typical alarms generated during testing onEDFC at New Tundlawith auto reset ELD are givenbelow:

[observe fault message generation when sup-ply is withdrawn from faulty conductor]

9 Summing Up

1. Earth Leakage of power supply leads to unsafeside failures of signalling system. Reduction ofMTTR (Mean Time To Repair) of earth faults

reduces the chances of having unsafe failures.

2. MTTR of leakage faults can be reduced by ap-plying data analytics to the data captured bydata logger from ELD & station signalling andidentifying the faulty conductor instantaneously.

3. There is need to relook at the Spec. of ELDmade 20 years back and modify it suitable topresent needs. Three modifications are sug-gested

i. Providing auto reset facility

ii. Providing leakage value in soft form todata logger

iii. Ability to work on 24 V DC

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Figure 11: Typical potential free contacts of relays and ELD for 24 V DC external supply

Station Fault Message1 VRBD Earth leakage occurred on

24V EXT-S/Z supply at11:27:28.Check for fault in the con-ductor of 01CTPR UP at11:27:26

2 VRBD Earth Leakage AppearedIn 24V EXT-S/Z Supply

3 VRBD Earth leakage occurredon110VSIG-S/Zsupply at13:16:08.Check for fault in the con-ductor of 9HECR UP at13:16:06

Table 3:

4. Railways must classify the earth leakage severityby the value of leakage and keep the process inplace for the rectification.

The information / views expressed in this pa-per is of the authors and are based on theirexperience. Comments / observations may besent to the author at sambireddy@efftronics.com

Station Fault message1 New

TundlaEarth leakage occurred on110 V DC Pointsupply(ELD)at 13:49:16Check for fault in the con-ductor of 231NWR UP at13:49:15

2 NewTundla

Earth leakage disappeared on110 V DC Pointsupply(ELD)at 13:49:22Check for fault in the con-ductor of 231NWR DN at13:49:21

Table 4:

Shri B. Sambi Reddy,IRSSE, Voluntarily re-tiredas CSTE / CN /ECOR in2007. Work-ing as ChiefEngineer in EFFTRONICSSystems PVT. LTD. fromthe last 10 years.

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