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Procedures

NPR 751 Calculating Minimum Warning Time

This document describes the procedure for calculating Minimum Warning Time (MWT).

Version 1.0, 29 April 2017

Description

This document describes the procedure for calculating Minimum Warning Time (MWT).

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Introduction

Minimum Warning Time (MWT) is the minimum time required for a Lookout or an Automatic Track Warning System (ATWS) to warn workers on track about approaching rail traffic.

MWT must be calculated using three key components:

  • the See time or Detect time:
    • a minimum of two seconds for the time it might take a Lookout to see approaching rail traffic (See time), or
    • a minimum of seven seconds for the time it might take an ATWS to detect approaching rail traffic (Detect time).
  • the Move time is the time taken for all workers to move themselves and their equipment to a safe place, and
  • the Safe time is the time all workers and equipment are in a safe place before rail traffic enters the worksite, a minimum of ten seconds.
Note
Note

An additional five seconds warning time must be added to the MWT if using an additional Lookout.

Protection Officers must use an approved Minimum Warning Time Calculator, Sighting Distance Calculator or Sighting Distance Table when determining:

  • the sighting distance required by a Lookout, or
  • the distance an ATWS sensor must be placed from the worksite.

If a track speed or a MWT is not listed in the following table use the next highest value shown.

Maximum track speed km/hSighting distance in metres to provide minimum warning time of:

15 sec

20 sec

25 sec

30 sec

35 sec

40 sec

45 sec

160

670

890

1115

1335

1560

1780

2000

150

625

835

1045

1250

1460

1670

1875

140

585

780

975

1170

1365

1560

1750

130

545

725

905

1085

1265

1445

1625

125

525

695

870

1045

1220

1390

1565

120

500

670

835

1000

1170

1335

1500

115

480

640

800

960

1120

1280

1440

110

460

615

765

920

1070

1225

1375

105

440

585

730

875

1025

1170

1315

100

420

560

695

835

975

1115

1250

95

400

530

660

795

925

1060

1190

90

375

500

625

750

875

1000

1125

85

355

475

595

710

830

945

1065

80

335

445

560

670

780

890

1000

70

295

390

490

585

685

780

875

60

250

335

420

500

585

670

750

50

210

280

350

420

490

560

625

40

170

225

280

335

390

445

500

30

125

170

210

250

295

335

375

25

105

140

175

210

245

280

315

20

85

115

140

170

195

225

250

15

65

85

105

125

150

170

190

Calculating Minimum Warning Time

Protection Officer

  1. Calculate the required:
    1. See Time or Detect Time, and
    2. Move Time, and
    3. Safe Time.
  2. Add the three numbers to give the MWT.
  3. Add five seconds to the MWT if an additional Lookout is used.

Calculating Minimum Sighting Distance

Protection Officer
  1. Establish the maximum track speed for the affected portion of track.
  2. An approved sighting distance calculator or sighting distance table to calculate the MSD.
Warning
Warning

As the duration of MWT increases, the accuracy of sighting distance becomes more critical.

Protection Officers must take care when estimating MSD.