Can Prime-Power and Standby DG Sets Use the Same RECD Specification?

Purchase teams love a single specification. One document, one approved vendor list, one set of spares, one training session for the maintenance crew. So when a site has two kinds of generator, one that runs most of the day because the grid is unreliable and one that waits for emergencies, the natural question is whether both can be covered by the same RECD specification.

The short verdict: they can often share a device family, and many clauses of the specification can be identical, but the parts that deal with sizing point, service intervals, monitoring and maintenance access usually need to differ. Copying a standby specification onto a prime set, or the other way round, is a common source of trouble later.

This article sets out what prime and standby actually mean, why the same engine carries two power figures, and then splits a typical RECD specification into the clauses that can stay shared and the ones that should change with duty. It ends with guidance for sites that want to standardise across a mixed fleet.

What “Prime” and “Standby” Mean

Generator ratings in India commonly follow the categories defined in ISO 8528-1, the standard on application, ratings and performance of generating sets. Cummins gives a clear summary in its note on understanding ISO 8528 generator set ratings. In broad terms:

Rating category	Intended use	Typical conditions attached
Emergency Standby Power (ESP)	Backup during utility failure	Limited annual running hours, variable load with an average well below the rating, no overload allowance
Prime Running Power (PRP)	Main source of power where the grid is absent or unreliable	Unlimited annual hours with variable load, average load limited over a 24-hour period, a modest short-term overload often allowed
Limited-Time Running Power (LTP)	Planned running at constant load for limited hours, such as peak shaving	Annual hours capped, constant load permitted
Continuous Operating Power (COP)	Base-load supply at constant output	Unlimited hours at a constant load

The exact limits differ between makers and should be read from your set’s documentation. What matters for this article is the pattern: a standby set is expected to run relatively few hours at varying load, while a prime set may run thousands of hours a year. Our article on standby vs prime power generators covers the buyer’s side of the distinction.

One Engine, Two Numbers

Look at a generator data sheet and you will often see two figures: a standby rating and a slightly lower prime rating. The engine is the same. The difference reflects how much power the maker is willing to warrant for each duty. A standby rating is commonly around 10% higher than the prime rating for the same set, though the margin varies.

This has a direct consequence for an RECD:

• Maximum exhaust flow. A set that may be run up to its standby rating will produce more exhaust at its peak than the same set limited to its prime rating. The device and the exhaust route must handle the higher figure if the set will ever reach it.
• Engine power band. CPCB’s System and Procedure for Emission Compliance Testing of RECDs works in terms of engine power, and devices are approved for power ranges. If a set sits near the edge of a range, the rating used in the specification can decide which device family applies. Use the engine’s gross mechanical power at the highest rating the set will actually be run to.

Our guide on how to read a generator kVA and kW rating explains the relationship between set rating and engine power.

How the Two Duties Differ from the RECD’s Point of View

Beyond the rating itself, prime and standby sets tend to live quite different lives. Here is how those lives look to the device in the exhaust.

Aspect	Typical standby set	Typical prime set
Annual running hours	Tens to a few hundred	Thousands
Typical run length	Short to moderate; many test runs	Long, often many hours
Typical load	Often light, especially during tests and night outages	Often moderate to high and sustained
Exhaust temperature	Frequently low	Usually healthy
Passive regeneration	Often the main concern	Usually reliable
Soot reaching the filter per year	Lower total, but deposited under poor conditions	Higher total, burned off steadily
Ash reaching the filter per year	Low	Much higher, because ash accumulates with hours and oil burned
Downtime tolerance	High; the set can be taken out of service easily outside outages	Low; stopping may halt production
Cost of a failure	Missed backup during an outage	Lost production or service

The two columns point in opposite directions. The standby set’s main risk is a filter that does not regenerate. The prime set’s main risk is a filter that fills with ash, combined with a duty that leaves little time to service it.

Clauses That Can Stay Shared

Many parts of an RECD specification do not depend on duty. These can be written once and used for both kinds of set.

Regulatory and approval

• The device must hold valid CPCB approval covering the engine’s power. Our guide on how to read an RECD test certificate explains what to check.
• Documentation for registration and inspection must be supplied.
• Installation must be carried out by the approved vendor or its authorised partner.

Engine compatibility

• Total exhaust backpressure, including the device with its normal soot load, must stay within the engine maker’s limit at the highest rating the set will run to.
• The vendor must review the engine’s condition before fitment and report any issues that would affect the device.

Construction and safety

• Housing material, insulation, guarding and labelling to stated standards
• Lifting points and supports suitable for the site
• Access for inspection of sensors and joints

Monitoring basics

• Backpressure and inlet temperature measurement
• Local display and, where required, remote data access
• Alarm on high backpressure

Warranty and documentation

• Drawings, manuals and commissioning records
• Warranty terms and spares list

These shared clauses make up most of the document by length. They are what a purchase team can standardise with confidence.

Clauses That Should Change with Duty

The differences sit in a smaller set of clauses, but those clauses decide whether the device works well over its life.

Sizing basis

Standby: Size for backpressure at the standby rating if the set may ever reach it during an outage. Pay particular attention to regeneration at the light loads that dominate standby running.

Prime: Size for backpressure at the highest load the set reaches in normal operation, including any short-term overload allowed under its rating. Regeneration is usually less of a concern, but confirm with load data.

Catalyst and regeneration approach

Standby: Favour a catalyst formulation and installation that support regeneration at low exhaust temperatures. Specify placement close to the engine and insulation. Require a planned loaded exercise run, perhaps monthly or fortnightly, as part of the maintenance plan. Our post on whether an RECD suits a DG set that runs only occasionally discusses this duty in detail.

Prime: A standard catalyst approach is often suitable. The specification should focus on durability under long hours and repeated heating.

Ash management and cleaning interval

Standby: Ash builds slowly because annual hours are low. Cleaning intervals measured in years are plausible, subject to monitoring.

Prime: Ash builds with every hour and every litre of oil burned. The specification should require the vendor to state the expected cleaning interval in running hours at the site’s load and oil consumption, and to describe the cleaning process, the time it takes and who does it. A prime set running several thousand hours a year may need ash cleaning far sooner than a standby set of the same size.

Maintenance access and downtime

Standby: The set can usually be taken out of service for a day for maintenance, provided the outage risk is managed.

Prime: Every hour off-line may cost production. The specification should require:

• A design that allows filter removal and refitting within a stated time
• Clear access for lifting the filter elements
• An option for a spare filter module, so a cleaned module can be swapped in while the other is sent for cleaning
• Coordination of RECD service with the engine’s own service schedule

Monitoring depth

Standby: Monitoring should catch slow soot build-up across many short or light runs, so a record of every run and a trend over weeks is important. Remote alerts help because the set may be unattended for long periods.

Prime: Monitoring should catch gradual ash build-up and sudden changes that indicate an engine problem. Integration with the plant’s SCADA or building management system is often worth specifying.

Warranty terms

Standby: Warranty periods stated in years are natural.

Prime: Ask for warranty terms stated in both years and running hours, so that it is clear what happens when a set accumulates many hours quickly. Our article on whether installing an RECD affects the generator’s OEM warranty covers the engine side.

The Split Specification at a Glance

Clause	Shared	Standby version	Prime version
CPCB approval for engine power	Yes
Backpressure within OEM limit with normal soot load	Yes	At standby rating	At highest operating load plus allowed overload
Construction, safety, documentation	Yes
Catalyst and regeneration		Low-temperature focus, exercise runs mandated	Durability focus
Placement and insulation		Close to engine, insulated	As layout permits
Ash cleaning interval		In years, reviewed by monitoring	In running hours, stated by vendor
Service access and downtime		Standard	Fast removal, spare module option
Monitoring	Basic shared	Per-run records, remote alerts	Trend for ash, SCADA or BMS link
Warranty		Years	Years and running hours

When a Set Changes Duty

Sets do not always stay in the role they were bought for. A standby set may become a prime set when a site moves to an area with poor supply, or when the grid connection is delayed. A prime set may become a standby set when a reliable grid connection finally arrives.

Either change should trigger a review of the RECD:

• Standby to prime. Annual hours rise sharply. Ash builds faster, service access becomes more important and the warranty position may change. A device chosen with low-temperature regeneration in mind will usually still work, but the cleaning interval and service plan must be rewritten.
• Prime to standby. Hours fall and load becomes lighter and more intermittent. Regeneration becomes the main concern. Exercise runs and placement need attention.

A set that carries a standby rating but is actually run as a prime source, sometimes for years, is also common. Apart from the engine risks, its RECD specification may be wrong. If your logs show high annual hours on a set rated for standby use, review both the rating and the device.

Sites That Have Both

Many Indian sites run a mixed fleet. Here are three typical arrangements and how to approach the specification.

Factory with captive prime power and a standby set

A plant in an area with poor grid supply runs one set almost all day and keeps a second set for backup and maintenance cover. The two may be identical machines.

Approach: Use one device family and shared clauses for both. Write the duty-specific clauses separately. Consider whether the sets can rotate duty, which evens out hours and ash loading and gives each filter a chance to rest. If they rotate, both devices should be specified to the prime version’s service and access clauses.

Data centre with multiple standby sets

A data centre has several large sets, all standby, with frequent testing and rare long runs. See RECDs for data centres.

Approach: One specification can cover all sets, built on the standby version. Pay close attention to test routines, which may be the bulk of running hours. Loaded tests through a load bank help both engine and filter.

Hospital with critical and non-critical sets

A hospital may have sets that serve critical loads and run only during outages, plus sets that run for longer periods to cover the main building when supply is poor. See RECDs for hospitals.

Approach: Treat each set by its actual duty, not its label. Sets with high running hours get the prime version of the service clauses even if their nameplate says standby.

A Worked Example: Two Sets at a Pharmaceutical Plant

This is an illustrative case. A pharmaceutical plant in Haryana has two identical 1000 kVA sets. Grid supply in the area is unreliable during summer.

What the logs showed over a year

Measure	Set 1	Set 2
Annual running hours	About 3,400	About 260
Average load while running	About 62%	About 28%
Median run length	About 7 hours	About 25 minutes
Role in practice	Prime source during summer and evening shortages	Backup, monthly tests, maintenance cover

How the specification was built

• Shared: one device family with CPCB approval covering the engine power; identical construction, safety and documentation clauses; the same backpressure limit and calculation method.
• Set 1 (prime duty): a service plan stated in running hours, with the vendor estimating the first ash cleaning on the basis of the engine’s oil consumption; a spare filter module held on site so cleaning would not stop production; monitoring linked to the plant’s SCADA; warranty stated in years and hours.
• Set 2 (standby duty): placement on the canopy roof with insulated pipe to keep inlet temperature up; monthly tests changed from 15 minutes at no load to 45 minutes at about 60% load; remote alerts for backpressure trend.
• Rotation: after the first year, the plant agreed to rotate the prime role between sets every quarter, which shares the hours, keeps both engines healthy and gives each filter a lighter year.

Pharmaceutical sites also need to handle these changes through their quality systems; our article on RECDs for pharmaceutical plants and change control explains how.

Using the Hours Meter as a Specification Tool

The simplest piece of evidence about a set’s real duty sits on its control panel. The hours meter, read at two dates a year apart, tells you annual running hours. Combined with the controller’s start counter, it also gives you average run length.

Annual hours	Starts per year	Average run length	Likely real duty
Under 100	50 or more	Under 2 hours	Standby, test-dominated
100 to 500	Varies	1 to 4 hours	Standby with significant outages
500 to 2,000	Varies	Several hours	Limited prime or heavy standby use
Over 2,000	Varies	Many hours	Prime in practice

These bands are a rough guide for sorting a fleet, not formal categories. A set in the top row needs the standby version of the duty clauses. A set in the bottom row needs the prime version, whatever its nameplate says. Sets in the middle rows deserve a closer look at their load records before the specification is finalised. Our article on how to check a used generator’s engine hours explains how to read and trust the meter.

Common Misunderstandings

“Prime sets need a bigger RECD.”

Not necessarily. The engine and its exhaust flow at the highest load reached decide the device size. A prime set limited to its prime rating may actually see a lower peak flow than a standby set allowed to its standby rating. The difference between duties lies more in service planning and durability than in size.

“Standby sets barely run, so the RECD is a formality.”

A standby set’s light, short running is one of the hardest duties for passive regeneration. Its specification needs as much care as a prime set’s, just in different places.

“One specification is always cheaper.”

Shared clauses save effort. But a specification that ignores duty can lead to devices that clog on standby sets or need unplanned downtime on prime sets, both of which cost more than writing two short duty sections.

“The rating on the nameplate tells you the duty.”

It tells you the duty the set was sold for. The logs tell you the duty it actually performs. Specify for the logs.

A Checklist for Purchase Teams

Before issuing an RECD tender for a site with prime and standby sets:

1. List every set with its rating, engine model and engine power.
2. Pull a year of running hours, loads and run lengths for each.
3. Classify each set by its actual duty.
4. Write the shared clauses once.
5. Write duty-specific clauses for sizing basis, regeneration, ash and cleaning, access, monitoring and warranty.
6. Ask vendors to respond clause by clause, stating their approach for each duty.
7. Check that each device offered is approved for each engine’s power.
8. Agree how the specification will be reviewed if a set changes role.

Our guide to choosing CPCB approved RECD vendors and the article on RECD installation timelines help with the tender stage, and the environmental compliance checklist for generator owners covers wider obligations.

Frequently Asked Questions

Can a prime power DG set and a standby DG set use the same RECD model?

Often yes, if both engines fall within the same approved power range and the device suits each set’s exhaust flow and backpressure. The service plan, monitoring and access arrangements should still differ to match each duty.

Which rating should be used to size an RECD?

Use the highest load the set will actually reach in service. For a set that may run up to its standby rating during outages, that is the standby figure. For a set limited to prime duty, it is the prime rating plus any allowed short-term overload.

Why does a prime set’s RECD need cleaning more often?

Ash from lubricating oil accumulates with running hours and does not burn off. A set running thousands of hours a year collects ash far faster than a standby set.

Is passive regeneration a problem on standby sets?

It often is, because standby sets tend to run briefly and at light load, especially during tests. Loaded exercise runs, placement close to the engine and insulation help.

What happens if a standby-rated set is used as a prime source?

Apart from engine risks, its RECD may be under-specified for ash cleaning, service access and monitoring. Review both the rating and the device.

Can identical sets rotate between prime and standby duty?

Yes, and it can help by sharing hours and ash loading. If they rotate, specify both devices to the more demanding service and access clauses.

Same Family, Different Instructions

Prime and standby sets can share an RECD family and most of a specification. Where they part ways is in the clauses that decide how the device lives: what load it is sized for, how it regenerates, how ash is managed, how quickly it can be serviced and how closely it is watched. Write those clauses for the duty each set actually performs, review them when duty changes, and a single tidy specification can serve a mixed fleet well.

Aceget writes RECD proposals around each set’s real duty and supplies CPCB type-approved devices for generators from 25 kVA to 2500 kVA. For the size in this article’s example, see the 1000 kVA DG set RECD; larger prime installations can refer to the 2000 kVA DG set RECD. To review a mixed fleet, contact Aceget or browse the retrofit emission control device catalogue.