What is the ABB CT-D range? CT-D is ABB's family of 17.5 mm DIN-rail electronic timing relays, built to IEC 61812 and IEC 60947-5-1, covering on-delay, off-delay, star-delta and multifunction/multi-range operation with DIP-selectable ranges from a fraction of a second to hours. The family splits into single-function models (CT-ERD on-delay, CT-AHD off-delay), a multifunction model (CT-MFD) that covers several functions and ranges in one part, and dedicated star-delta variants (CT-VWD, CT-YDD, CT-SDD) — so a panel builder standardizes on one enclosure cutout and picks the function per application. This review works through the functional split inside CT-D, timing ranges and accuracy, contact ratings, mounting and supply, and where the range sits against the earlier CT-S and against Schneider's Zelio Time equivalents.
What the CT-D Range Includes
ABB groups CT-D by function rather than by a single universal part. CT-ERD is the on-delay-only member: apply power, wait the set time, the output changes and stays until power is removed. CT-AHD is its off-delay counterpart, changing the output on trigger and reverting after the set time following removal of the trigger. Both are single-function, single-range-select devices aimed at panels where the function never needs to change in the field.
CT-MFD is the multifunction model. One housing, several functions (on-delay, off-delay, single-shot, symmetrical/asymmetrical flasher among them) and several time ranges selected by rotary switches, with the actual delay set by a potentiometer or fine-adjust dial. It replaces multiple single-function parts on a stock shelf with one SKU, at the cost of a slightly busier front panel — two rotary switches and a dial instead of one.
CT-VWD, CT-YDD and CT-SDD are the star-delta members, described in their own section below. CT-S is the earlier generation that CT-D superseded; it still turns up in older panels and on used-equipment listings.
| Criteria | CT-ERD / CT-AHD | CT-MFD | CT-VWD / CT-YDD / CT-SDD |
|---|---|---|---|
| Function | Single: on-delay or off-delay only | Multiple, rotary-selected (on-delay, off-delay, flasher, single-shot, etc.) | Star-delta, two outputs (star + delta) with dead-time |
| Time ranges | One or two fixed ranges per model | Multiple ranges on one part, typically 0.1 s to 100 h band | Separate run-up range and short transition range |
| Outputs | 1 c/o or 1 c/o + status LED | 1-2 c/o | 2 outputs (star contactor, delta contactor) |
| Typical use | Fixed, known delay that will not change | Mixed-function panel stock, spares standardization | Star-delta motor starting |
CT-MFD: The Multifunction, Multi-Range Model
CT-MFD's value is in the spares bin, not the individual delay. A control panel shop that stocks CT-ERD, CT-AHD, an interval timer and a flasher separately can often replace all four SKUs with CT-MFD units, set differently at commissioning. The trade-off: a multifunction part costs more per unit than the cheapest single-function equivalent, and the extra rotary switches are one more thing a commissioning technician can set wrong. Multifunction and multi-range timing relays covers the function list and range bands in more detail.
What we see in the field: panels with CT-MFD installed as an on-delay-only unit, function switch never touched again after commissioning. That is a legitimate use — it just means the panel paid for flexibility it never exercises. Where the delay genuinely needs re-tuning after startup (seasonal HVAC schedules, for example), the extra cost earns its keep.
CT-ERD and CT-AHD: Dedicated On-Delay and Off-Delay Timers
CT-ERD times from power-on: the output changes state only after the set delay elapses, and holds that state as long as power stays applied. It is the more common of the pair — sequencing a second load after a first has stabilized, or holding an alarm output back long enough to ride through a normal startup transient. See on-delay timing relay for the function in isolation.
CT-AHD is control-voltage off-delay: it changes state immediately on trigger, then reverts the set time after the trigger is removed, but only while its own supply stays connected. That distinction matters. A control-voltage off-delay stops timing the instant its supply drops, so it cannot hold a fan running after a full panel shutdown. A true off-delay needs stored energy (a capacitor bank inside the relay) to keep timing with no external supply at all — check which behavior a given CT-AHD variant provides before specifying it for a run-on function tied to main power loss. Off-delay timing relay explained covers this distinction against true off-delay.
CT-VWD, CT-YDD and CT-SDD: Star-Delta Timing
All three drive star-delta motor starting, but split the job differently. The timer closes the star contactor, times the motor run-up — typically a few seconds up to around 30 s depending on the motor and load inertia — opens star, holds a short transition (dead-time) of tens of milliseconds, then closes delta. The transition exists purely to stop the star and delta contactors bridging phase-to-phase during the changeover; skip it or set it too short and the contactors overlap. Star-delta timing relays for motor starting covers the run-up/transition split in more depth, and contactors for the switching side of the same starter.
CT-VWD and CT-YDD both perform the two-output star-delta sequence; CT-SDD is the compact/simplified member of the group aimed at smaller motors where a single fixed transition setting is acceptable rather than an adjustable one. Which of the three fits a given panel depends on motor size and whether the transition needs field adjustment — check the current ABB CT-D datasheet for the exact model split before specifying, since ABB has consolidated star-delta SKUs across catalog revisions.
Timing Ranges, Accuracy and Contact Ratings
Across CT-D, timing range is set with a two-stage control: a rotary or DIP switch picks the range band (for example 0.1-1 s, 1-10 s, up to a multi-hour band on CT-MFD), and a potentiometer or fine dial sets the value within that band. Repeat accuracy — how close successive timing cycles land to each other at the same dial setting — typically runs in the low single-digit percent of the set value on electronic CT-D members, well tighter than the pneumatic timers CT-D and CT-S replaced.
Formula: repeat accuracy error — Source: IEC 61812, timing accuracy definitions
e = (tmax - tmin) / tset x 100
| Symbol | Description | Unit |
|---|---|---|
| e | Repeat accuracy error | % |
| tmax | Longest measured delay across repeated cycles | s |
| tmin | Shortest measured delay across repeated cycles | s |
| tset | Delay value set on the dial | s |
Contact rating on CT-D is modest by design — a change-over contact rated a few amps at 250 V AC, enough to drive a contactor coil or a PLC input, not enough to switch motor current directly. That is normal for a control-circuit timer under IEC 60947-5-1; it is not a fault to design around, it is the category. Timing ranges and accuracy explained goes through what the range/accuracy numbers mean when comparing two datasheets side by side.
Mounting, Supply and Where CT-D Fits in a Panel
CT-D is a 17.5 mm modular DIN-rail device, the same footprint discipline ABB and Schneider both use across their compact timer lines, so swapping a CT-ERD for a CT-AHD later changes function without changing the enclosure layout. Supply is multi-voltage on most CT-D members — a single part accepting a band of AC and DC control voltages — which cuts the number of voltage-specific SKUs a panel shop has to stock. Socket-mount variants exist for panels that prefer plug-in replacement over hard-wired terminals; fixed-terminal versions save panel depth where socket clearance is tight.
Where CT-D sits against the older CT-S: CT-S is still supported and still shows up in maintenance work, but new panel designs specify CT-D for the tighter repeat accuracy and the multifunction option CT-S does not offer in the same form. Against Schneider's Zelio Time RE17/RE22, the two ranges cover the same functional ground — on-delay, off-delay, multifunction, star-delta — as compact multi-voltage timers with change-over outputs; the practical differences are module width, how many functions combine on one part, and star-delta transition granularity. How to select a timing relay walks through picking between single-function and multifunction on cost and stocking grounds, and the timing and control relays collection carries both ranges for side-by-side specification.
For the broader functional background behind every CT-D model, see the timing relay engineering guide.
Frequently Asked Questions
What functions does the ABB CT-D range cover?
On-delay (CT-ERD), off-delay (CT-AHD), multifunction/multi-range (CT-MFD), and star-delta motor starting (CT-VWD, CT-YDD, CT-SDD), all as 17.5 mm DIN-rail electronic timers built to IEC 61812 and IEC 60947-5-1.
What is the difference between CT-MFD and the single-function CT-ERD/CT-AHD models?
CT-ERD and CT-AHD each perform one function at one or two fixed ranges. CT-MFD selects between several functions and several time ranges on the same physical part via rotary switches, at a higher per-unit cost and a busier front panel.
How do CT-VWD, CT-YDD and CT-SDD differ from each other?
All three run the star-delta sequence — star contactor, run-up timing, transition dead-time, delta contactor — but split by adjustability and motor size. CT-SDD is the simplified/compact member for smaller motors; CT-VWD and CT-YDD offer the fuller two-output, adjustable-transition sequence. Confirm the current split against ABB's datasheet, since catalog SKUs get consolidated over time.
What repeat accuracy can I expect from a CT-D timer?
Electronic CT-D members repeat in the low single-digit percent of the set value across cycles, tighter than the pneumatic timers the range replaced. Exact figures vary by model and range band — check the specific datasheet rather than assuming one number across the family.
Are CT-D timers a straight swap for the older CT-S range?
Mechanically they share the 17.5 mm DIN-rail format, but function coverage and accuracy differ between generations. Treat a CT-S-to-CT-D replacement as a spec check, not an assumed drop-in, particularly for star-delta transition timing.
Conclusion
CT-D's real structure is a function split, not a single product: CT-ERD and CT-AHD for a delay that never changes, CT-MFD when one SKU needs to cover several functions across different builds, and CT-VWD/CT-YDD/CT-SDD for star-delta starting where the transition setting is the safety-critical number. Match the split to how the panel is actually built and stocked, not to whichever part happens to be on the shelf. Stoklink carries the CT-D range alongside Schneider Zelio Time in the timing and control relays collection and the soft starters that cover the same motor-starting duty at higher motor ratings.