Touch Potentials – Typical Practice Is Not Necessarily Safe

Traditional earth fault limits, earth loop impedances and protection clearance time ‘rule of thumb’ do not always result in safe or compliant systems.

Find out more below or download as a PDF version.

A Typical Underground Example

Touch Potentials

 

A typical 1000V underground supply system commonly has a 5A earth fault limit. The cables feeding outlet control and the load are often protected by earth continuity relays with a 45Ω pilot earth loop impedance limit. Allowing for the pilot resistance of the installed cable length, could see the total return earth impedance for an earth fault in the load as high as 75Ω.

Worst case touch voltage is: 75/(115+75)*577 = 228V 

Total clearance time for outlet E/L:

  • Earth leakage relay 50 msec (instantaneous)
  • Interposing relay delay 20 msec
  • Breaker/contactor delay 130 msec

= a total clearance time of 200 msec

ASNZS4871

 

Not compliant or safeThe scenario is typical of underground practice as the key operating parameters are consistent with values allowed in standards

  • 5A earth fault limit
  • 45 Ohm earth return impedance limit

The potential touch voltage clearance times are to the right of the safe area of the wet area curve (Lp) in AS/NZS4871.

 

What you need to do

The 2012 changes to AS/NZS4871 are more significant than generally appreciated. Traditional parameter values available under standards are not necessarily ‘safe’ when configured into a practical system.

Removal of prescriptive limits on key parameters including earth fault limits, trip settings and clearance times, requires all design settings to be examined from first principles.

Regardless of whether they fall below the wet area curve or not, all protection parameters should be justifiable as being as low as reasonably practical.

How to get there

  • Complete an audit of your system against the requirements of AS/NZS4871
  • Carefully consider the fundamental parameters including:
    • Earth fault limitation
    • Return earth impedance limit
    • Tripping ratio and total clearance times
  • Review underground substations for compliance against AS/NZS4871 and Safety Bulletin SB11-04 (variable speed drives and fitment of wideband earth leakage)

How Ampcontrol can help

Not sure where to start or need some help with an audit? Ampcontrol have a range of services available to help you.

Electrical engineering support: Ampcontrol’s electrical engineering team have the experience and expertise to understand and evaluate your electrical system. We can undertake protection and electrical distribution studies and provide electrical design advice.

Onsite support: Ampcontrol’s underground and HV service teams are available to conduct AS/NZS4871 audits. Our trained service technicians and electrical engineers audit your site, provide standardised documentation of audit findings, and provide recommendations for any issues identified.

Training: We provide practical training modules including how to set and configure electrical protection equipment as well as high voltage maintenance program requirements. Our training packages are custom designed to your site installation and include reference materials. Training can be conducted on a scheduled basis to ensure your staff remain up to date with requirements.

 

High Integrity Isolation System

Automated isolation systems are becoming widely used to provide high integrity isolation solutions that improve productivity and reduce manual handling.

Traditional isolation procedures in underground coal mines require connection and disconnection of high voltage electrical cables to be carried out by trained and competent electricians who have obtained a switching permit to perform work. These personnel then need to travel to the isolation location, which is commonly remote from the work location, perform the isolation and confirm its success. Although the
process provides a safe methodology for electrical isolation it comes at the cost of production and potential manual handling issues due to the size and weight of the cables.

To counteract these issues and increase efficiency, automated isolation systems are becoming widely used. These engineered solutions routinely employ automation to simplify and control plant operation and
can be applied to procedural safety and operational management. These automated systems provide immediate productivity improvements by removing the need to use a trained and competent electrician to complete the isolation, enabling isolations to be performed by operators and maintenance personnel, it also provides a decrease in manual handling of heavy cables and a minimisation of the time and travel associated with conventional manual isolation procedures.

High Integrity Isolation: A Case Study

This underground mine had a distributed Remote Isolation System, which allows for isolations to be initiated and confimred from numerous locations. This Remote Isolation System is used for minor short duration tasks. Through a risk management process, the mine site determined that isolations could be effected using a specially designed High Integrity Isolation System (HIIS) for more complex, longer duration tasks.

Ampcontrol designed a HIIS which opened multiple switch devices within the power circuit to ensure an effective isolation and incorporated Electrical, Electronic, Programmable Electronic (E/E/PE)
components in its control system. With this being the case, the HIIS was designed, tested, installed and commissioned in accordance with the functional safety principles and lifecycle outlined in AS/NZS61508.

This change in isolation philosophy provided a single high integrity isolation point, located near the 11kV longwall face conveyor, which effectively and safely isolates the face conveyor and associated drives and allows the fulfilment of multiple personal danger padlocks to secure the isolation.

The HIIS removes the requirement for electricians to perform isolations by physically removing heavy plugs. This allowed isolation and restoration of power to be performed by operators and maintenance personnel, saving valuable production time while achieving an effective and safe isolation.

Find out more about this system below or download as a PDF. 

The Scope of Work

The mine site developed a functional specification defining the system requirements including:

  • The HIIS was to be a single isolation point for the face conveyor motors
  • The requirement for a E/E/PE system (Electrical/Electronic/Programmable Electronic)
  • Operational with the system energised or de-energised
  • The maintenance tasks they wanted to undertake utilising the HIIS
  • Their tolerable risk level
  • That the system would be a High Demand system
  • Maximum proof test interval

The System Design

After the scope was defined, Ampcontrol developed a detailed design following their Functional Safety Management Plan which aligns with the requirements of AS/NZS61508. This included facilitating a detailed hazard/ risk assessment and Layer of Protection Analysis with the mine site as part of the process to meet the target specification of a dangerous failure rate of 1 in 100,000 years. The Layer of Protection Analysis showed that to achieve the desired safety integrity level the design required multiple independent E/E/PE systems to provide sufficient redundancy to achieve this target.

The final design consisted of three separate monitoring and control system layers to achieve the target failure rate, as shown in the Function Block Diagram (below) and subsequent Fault Tree Analysis. With the first layer being a SIL3 system and the second and third layers both being SIL1.

High Integrity Isolation System - underground mining

The SIL3 system provides the primary level of safety protection which is operated by redundant inputs with fault detection from the High Integrity Isolation Switch. The PLC then disconnects power to the conveyor and longwall motors by opening the associated contactors and main circuit breakers via safety relays. The switching sequence and timing is utilised to ensure the isolation procedure has been effected properly in the required timeframe and the final step in the process is to affix personal danger padlocks securing isolation. The system includes many safety related products including an Allen Bradley Guardlogix PLC, safety relays, cable fault monitoring relays and controllers.

In the event both the primary and secondary levels were to fail, a tertiary system is designed to disconnect the supply to the whole longwall system by opening the main upstream supply circuit breaker. This tertiary system is designed to SIL1 and consists of a signal line/ remote isolation system communication line to disconnect power upstream in the event any of the longwall contactors and main circuit breakers remain closed when they were commanded to open.

Following design and installation in accordance with AS/NZS61508, the system design and calculations were independently assessed for compliance and a certificate was provided by the independent third party.
Confirmation of the system operating as expected under all normal and abnormal conditions was undertaken by Ampcontrol’s trained staff using functional testing and the introduction of faults to confirm the adequacy of the design and manufacture and safe operation under all failure modes.

From these tests a list of any possible dangerous undetected faults were compiled into a proof testing document which was developed in such a way to allow for non-invasive proof testing at the periodic proof testing intervals as required by the SIL analysis to allow the mine to maintain the SIL claim.

This system is currently in operation and used for a range of maintenance tasks providing increased productivity and output by significantly decreasing isolation and restoration times at the mine site.