Guide · Train detection
SSDAC vs MSDAC: choosing the right digital axle counter
Single section and multi-section digital axle counters do the same safety job in different ways. This guide explains how they differ and how to choose the right one for block sections, sidings, point zones and station yards.
What both systems have in common
A digital axle counter proves a track section clear by counting the axles that enter and leave it. Detectors on the rail sense each wheel, and the system reports the section clear only when the two counts match and the equipment is healthy. Any fault, power loss or count mismatch drives the section to a safe, occupied state.
Both the single section digital axle counter (SSDAC) and the multi-section digital axle counter (MSDAC) work this way, and in India both are specified under RDSO/SPN/176. The difference lies in how much track one system covers and where the counting is evaluated.
Key differences at a glance
| Aspect | SSDAC | MSDAC |
|---|---|---|
| Coverage | One track section | Many track sections from one system |
| Detection points | One to four per section | Many per evaluator; INSYS DAC handles up to 40 |
| Where counting is evaluated | In the field units of each section | In a central evaluator, typically in the relay room |
| Typical applications | Block sections, sidings, terminal lines, point zones, automatic signalling with consecutive sections | Station yards, ladder layouts, complex point zones, automatic signalling territory |
| Interlocking integration | Vital relay output per section | Vital relay outputs plus a serial interface to electronic interlocking |
| Scaling up | Add another system for each new section | Add detection points and sections to the existing evaluator, within its capacity |
| INSYS product | SSDAC: SIL 4 compliant, assessed by Bureau Veritas Italy; RDSO approval in progress | DAC (MSDAC): built to be SIL 4 compliant, 2oo3 central evaluator; in development |
When to choose an SSDAC
Block sections between stations
A straightforward section with a detection point at each end is the classic SSDAC application.
Sidings and terminal lines
A single detection point proves a dead-end line or siding clear.
Isolated point zones
Three or four detection points cover a point zone without a central evaluator.
Distributed sites
Where sections are spread out and there is no convenient central equipment room, independent systems per section keep cabling simple.
When to choose an MSDAC
Station yards
Many interconnected sections are managed from one evaluator in the relay room, rather than many separate systems.
Ladder and crossover layouts
Shared detection points across adjacent sections suit multi-section evaluation.
Electronic interlocking
A serial interface passes the status of every section to the interlocking in one link.
Centralised maintenance
Diagnostics, event logs and resets for a whole yard sit in one place.
Questions to ask before you decide
- How many track sections and detection points does the layout need now, and in future?
- Is there a relay room or equipment room close to the sections?
- Which communication media are available: quad cable, optical fibre or both?
- Is the station on relay or electronic interlocking?
- What safety integrity level and approvals does the tender specify?
- How will maintenance staff reset, diagnose and log faults?
Many projects use both: SSDACs for block sections between stations, and an MSDAC for the station yard.
Not sure which fits your project?
Share your layout, number of sections and site conditions, and we will recommend the right configuration.
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