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COMPLIANT VS SAFE:

COMMON PRACTICE

AGAINST AS/NZS4871:2012

23rdELECTRICAL

ENGINEERING SAFETY

SEMINAR

WEDNESDAY 6thNOVEMBER

2013

TIM WYLIE

CHIEF TECHNOLOGY OFFICER

AMPCONTROL

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Copyright Ampcontrol 2013

1. What is compliant? What is safe?Understanding differences between voltage vs time exposure limits in 4871,

2067 & 3007.

2. Common practice (compliant?) examples against 4871We will explore a number of common scenarios for protection gradings and

settings against the latest revision of AS4871, and investigate simple

modifications that counter the traditional guidance and rules of thumb that can

be shown to markedly improve safety.

3. More than just an earth fault limitation?Damping the forgotten function of the NER.

4. Summary observations

PRESENTATION

OVERVIEW

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Is there a distinction between what is compliant and whatis safe?

Traditional earth fault limits in presently in common use

are compliant eg: 5A fault limit for 1000V systems inhazardous areas

Earth loop impedance limits set out in AS2081 arecompliant

Touch potentials resulting from compliant fault limits andloop impedances are not necessarily safe under the newrevision of 4871

OVERVIEWCOMPLIANT IS NOT NECESSARILY SAFE ?

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Any given parameter is considered to be compliant if

it is nominated as such in a Regulation, in an MDG or

in an applicable Australian Standard. The earth fault current limits set out in previous

revisions of AS/NZS2081 are generally considered

compliant.

The limits for earth loop impedance and relay trippingtimes in AS/NZS2081 are compliant.

WHAT IS

COMPLIANT ?

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AS/NZS4871.1:2012 relates the maximum duration ofhuman exposure to prospective touch voltages that do not

usually result in harmful physiological effects on any

person subjected to that touch voltage (i.e. safe).

WHAT IS SAFE ?

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A RECENT ADDITION TO

AS4871MAXIMUM DURATION OF 50Hz TOUCH VOLTAGE

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AS/NZS2067:2008 Clause 1.1 indicates

This Standard does not apply to the design and erection of any

of the following:

(C) Mine site electrical installations, or parts of such installations

Defines two curves (special and normal locations).

The special location applies where there is negligible additional

body resistance (bare hands/feet, minimal additional series

impedance.

The normal curve applies in dry situations where footwear

and additional series impedance (PPE, gloves etc) are

expected.

WHAT ABOUT AS2067 ?SUBSTATIONS AND HV INSTALLATIONS EXCEDING

1kV AC

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AS/NZS3007:2004 provides two curves sets

installations up to 1000V;

installations exceeding 1000V.

Both curve sets exceed the AS4871:2012 Lp curve

and in some instances the dry area L curve.

Using AS/NZS60479:2010, it can be demonstrated that

higher touch voltage exposures allowable underAS3007 were not intended for application in

underground wet area environments.

AS/NZS3007 CURVE

SETS ?

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COMPARISON OF

TOUCH POTENTIAL

LIMITS

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Each curve results from differing assumptions in clothing,

footwear, PPE and environment and so body current.

Which one is right ?Depends on the application

The definition of safe could be inferred from the

amalgam of applicable standards as the lowest voltage

versus time duration of all curve sets. The Lp curve ofAS4871:2012 closely approximates the minimum but

may not be achievable or necessary in all applications

WHY ARE THEY DIFFERENT ?WHICH ONE IS RIGHT ?

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BASIC TOUCH

POTENTIAL ANALYSIS

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TOUCH POTENTIAL

ANALYSISCONVENTIONAL TOUCH POTENTIAL ANALYSIS

For a 1000V system, phase to earth voltage is 577V, for 5A limitation

NER is ~115 Ohms

At 45 Ohm earth continuity limit, touch potential is:

577 x 45 / (45 + 115) = 162V (how hazardous is this?)

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DURATION OF 50Hz TOUCH

VOLTAGEAS/NZS4871

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At 162V, total protection clearance time (relay setting plus

contactor actuation time) must be ~100msec.

If the operating time of our protection exceeds 100msec we

must increase the NER impedance or reduce the return earthcontinuity limit accordingly.

The presence of an NER does not make the system safe.

Safety is reliant on active earth leakage protection.

If our active protection fails to operate we must restrict the continuoustouch potential to less than 25V to remain safe.

Active protection must properly resolve leakage current both in

terms of magnitude and frequency.

TOUCH POTENTIAL

ANALYSISCLEARANCE TIMES AT 50Hz

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EXAMPLE 1:

DUAL 45 OHM CASE

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DUAL 45 OHM CASETYPICAL DEVELOPMENT

ELECTRICAL SYSTEM

5A earth fault limit at substation, EC relays in sub andDCB both allow up to 45 Ohms pilot earth impedance

Total return impedance for a fault at the load may be

as much as 75 Ohms

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DUAL 45 OHM CASETYPICAL DEVELOPMENT

ELECTRICAL SYSTEM 5 Amp earth fault current limit, 35+40 = 75 Ohms earth

return impedance

Prospective touch voltage is 75/(115+75)*577 = 228V

Clearance time:

earth leakage relay 50 msec

interposing relay delay 20 msec

circuit breaker delay 130 msec the total clearance time is around 200 msec.

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VOLTAGEAS/NZS4871

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DUAL 45 OHM CASESUMMARY OBSERVATIONS

On one hand: scenario is arguably compliant with olderstandards revisions since the key operating

parameters are consistent with:

5A Earth fault limit45 Ohm earth return impedance limit

Typical of previously accepted practice

On the other hand: Prospective touch voltage

clearance times are to the right of the safe area underthe Lp curve in AS/NZS4871

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200msec clearance time for the Lp curve requires

110V touch potential limit

Can adjust the NER, E/C limit or both With 75 Ohms return earth impedance, the NER

value can be back calculated to limit the maximum

earth fault current to around 1.8A

With a tripping ratio of 10:1, experience indicates a180mA trip current may be impractical in many

applications we will come back to this.

DUAL 45 OHM

CASEAN ALTERNATE SOLUTION

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EXAMPLE 2:COMMON GRADING

SETTINGS

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TYPICAL GRADINGTYPICAL DEVELOPMENT

ELECTRICAL SYSTEM

Resultant touch voltage is 134V

Clearance time 400msec (E/L relay 250msec,

interposing relay 20msec, CB delay 130msec)

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VOLTAGEAS/NZS4871

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COMMON GRADING

SETTINGSAN ALTERNATE SOLUTION At 400msec clearance time, touch voltage reqd ~57V

Back calculate current limitation for 35 Ohm return

impedance as 1.8A (same as previous example)

What about 10:1 tripping ratio?Under 4871:2012 test current for E/L relay is 120% or 1.2:1

Typical tripping ratios in UK are 3:1, US are 2.5:1

Tripping ratio should be maintained high as possible,

but at 350mA trip (5:1) system would be safe

Increased trip current avoids sympathetic trips on

unfaulted outlets and relays still proven/tested to trip

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DAMPING FORGOTTEN

FUNCTION OF THE

NER

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LESS CURRENT IS BETTER

?WHY NOT REDUCE THE EARTH FAULT CURRENTLIMITATION FUTHER ?

In previous examples, the situation with regard to risk

from touch potential and delivered energy is

demonstrably improved as the fault current limit is

reduced

Touch potential hazard and the delivered fault energy

can both be reduced to zero if the NER impedance is

made infinite What is the upper limit for NER impedance ?

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NER AND COMMON MODE

DAMPINGINDUCTIVE FAULTS IN MOTOR OR TRANSFORMERWINDINGS

Capacitance to earth dominated by cabling Large fault inductances can be created in winding faults (several

Henry), common mode resonant frequency may approach 50Hz or a

harmonic frequency of 50Hz

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In a practical 11kV system with an inductive fault:

Earth fault limit 50mA, up to 40 times nominal insulation

stress

Earth fault limit 5A, 3 times nominal insulation stress

Earth fault limit 10A, 1.2 times (so acceptable)

In any practical system, the NER impedance can be nolarger than would afford sufficient damping to prevent

excessive insulation stress in case of an inductive fault.

NER AND COMMON MODE

DAMPINGA PRACTICAL EXAMPLE

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SUMMARYOBSERVATIONS

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Recent changes to AS/NZS4871 are more significantthan generally appreciated

Previously compliant elements are not necessarily

compliant when configured in a practical system Removal of prescriptive limits on key parameters (earth

fault limit, trip settings and clearance times) requires all

design settings to be examined from first principles

We must be able to justify all protection parameters(regardless of if they fall below the Lp curve or not) as

being as low as reasonably practical

SUMMARY OBSERVATIONS

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Complete an audit against AS/NZS4871

Carefully consider the fundamental system parameters:

earth fault limitation

return earth impedance limit

tripping ratio and total clearance times

There is an increasing need to review underground

substations:Compliance against AS/NZS4871

Safety Bulletin SB11-04 (variable speed drives & fitment of

wideband earth leakage, NER dissipation ratings etc)

SUMMARY OBSERVATIONSA PROACTIVE APPROACH

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Contacts

ResTech: Steve Clifton [email protected]

Ampcontrol: Tim [email protected]

ResTech Two Day Short Course

Principles and pitfalls associated with electrical protection in

earth fault limited systems

Day 1: Theoretical background and framework

Day 2: Investigation & real world case studies

Register your interest online at:

restech.net.au/earthfaultseminar

FURTHER ASSISTANCE &

INFORMATIONCONTACT RESTECH OR AMPCONTROL
mailto:[email protected]:[email protected]:[email protected]:[email protected]

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QUESTIONS?

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