RPZ testing explained

None of the state or utility documents read for this page use the letters RPZ. They use reduced pressure principle assembly, often shortened to RP. If your notice says RP, RPZ, or reduced pressure zone, match it to the label on the assembly and to the letter from your utility. This page does not choose the device for an address.

What the assembly is

Greenville Water’s Cross Connection Control Manual (PDF created November 10, 2025) defines a reduced pressure principle assembly (RP) as two independently acting, spring-assisted check valves with a hydraulically operating differential relief valve between the checks and beneath the first check. It has resilient-seated test cocks and resilient-seated or ball shutoff valves at each end. The manual says the assembly has to stay accessible for in-line testing, in a place that is never subject to flooding, and that it is approved for high-hazard cross connections. The same definition says the assembly needs approval from SCDHEC and Greenville Water. Tester cards are now issued by SCDES, as the program page describes. How those cards differ is on General vs Limited.

Spartanburg Water’s manual revised June 4, 2026 uses the same description and says the assembly needs approval from SCDES and Spartanburg Water. The file linked from Spartanburg Water’s cross-connection page on October 7, 2026 uses underscores in the name.

SCDES’s sample test form labels the assembly a reduced pressure backflow preventer and marks RP as ASSE #1013. The same form has a line for a reduced pressure detector assembly, RPDA, ASSE #1047. The form says many jurisdictions require their own form.

The January 1, 2026 approved-assembly list calls these reduced pressure principle assemblies (RPPA). It says the relief port must never be submerged or subject to a toxic environment, which prohibits a pit that cannot drain by gravity to the ground surface. It also says an RPPA is not acceptable in a vertical orientation unless the University of Southern California’s Foundation for Cross Connection Control and Hydraulic Research has approved that orientation. Assemblies used in South Carolina have to be on that list. What is backflow testing covers the list and the annual-test rule for every testable assembly, not only this one.

Where the state rule requires one

R.61-58.7(F) separates hazards:

• A high hazard, in section (4)(a), is a connection to a service or other water system that has or may have any material dangerous to health, handled under pressure or subject to negative pressure. Protection is an air gap or an approved reduced pressure principle backflow prevention assembly.
• Section (4)(b) says these assemblies are not installed where flooding is possible. That includes a pit or vault without a gravity drain to the ground surface that can pass more water than the relief valve discharges.
• Section (3)(b) treats residential lawn irrigation as a high hazard when the system adds chemicals, or when it is also connected to a water source that is not an approved public water system. That case has to meet section (4).
• Section (5) says fire-line sprinkler systems in the high-hazard category also have to meet section (4). The rule’s examples include antifreeze systems, foam systems, and systems charged from or tied into a pond, lake, stream, or any source other than the approved public supply. Other fire lines use an approved double check valve assembly.
• A low hazard, in section (2), is a different case. The minimum the rule names there is an approved double check valve assembly or a pressure vacuum breaker, except for the residential dual-check policy in section (3). An RP is not the minimum that sentence requires.

What Greenville Water’s manual adds

Section 5-1 says a reduced pressure principle assembly or an air gap is used for a high hazard. A double check valve assembly or a dual check is used for a low hazard. The manual does not name a pressure vacuum breaker for the high-hazard category.

Section 5-5 is only for the RP. If pressure swings make the relief valve discharge often enough to be a nuisance, the customer should add a check valve ahead of the assembly. Relief-valve drain piping has to keep an air gap of two times the relief-valve diameter, or 1 inch, whichever is greater, and the piping has to be sized to carry more than the relief valve discharges. The relief valve is never submerged. Underground installation is discouraged and is allowed only in unusual cases with a gravity drain to the surface that is large enough.

Section 6-2.1 says a high-hazard fire system — including antifreeze, foam injection, or a supply from a lake, pond, stream, or any source other than Greenville Water — needs a reduced pressure principle assembly on the connection to Greenville Water, as close as practical to the service connection and the property line. A low-hazard wet or dry sprinkler system uses a double check valve assembly.

Greenville Water’s backflow page says the utility does not install or test backflow preventers. Section 7 of the manual says a certified tester tests the assembly. A new testable assembly is tested at installation, before the customer uses it, and that report has to reach Greenville Water within 10 days of the water service starting. Existing assemblies are tested at least annually. Notice timing and the non-compliance fee are on the Greenville Water page. Inspection vs testing separates the utility’s site inspection from this test.

What Spartanburg Water’s manual adds

Section 9.1 says a high hazard requires a reduced pressure principle assembly, a pressure vacuum breaker for back-siphonage only, or an air gap. A low hazard requires a double check valve assembly or a double detector check assembly. Where that pressure-vacuum-breaker sentence and the state rule’s high-hazard sentence do not read the same way, use the notice Spartanburg Water sent for the account.

Section 9.4 repeats the relief-valve air gap of two times the relief-valve diameter or 1 inch, whichever is greater, and says the relief valve will never be submerged. Underground installation is discouraged and is allowed only in exceptional cases, one site at a time, where a gravity drain can be installed.

Section 9.5 says a high-hazard fire system, including antifreeze, foam injection, or a lake, pond, stream, or other outside source, needs a reduced pressure assembly or an air gap as close as practical to the service connection and the property line. A remote section that is only an antifreeze loop or a chemical injection point may be protected with a reduced pressure assembly at that chemical addition or at the water connection. A simple wet or dry system uses a double detector check assembly.

Section 8.4 says a private well has to be physically disconnected before a Spartanburg Water meter is turned on. If the customer keeps the well connected to that metered service, a reduced pressure backflow prevention device goes on the customer side of the meter.

Section 10.1 says a high-hazard residential lawn irrigation system needs a reduced pressure assembly, and that regulation requires an annual test of that device. A low-hazard residential lawn system is a dual check on a different schedule. That is not this test.

Section 10.2 says the customer uses a Spartanburg Water–approved SCDES-certified tester. The tester sends a report showing compliance within 7 days of the test. The notice tiers and the administrative fee are on the Spartanburg Water page.

What Anderson’s 2008 policy adds

The September 2008 Cross Connection Control Policy says a high hazard requires an air gap, a reduced pressure principal backflow prevention assembly, or a pressure vacuum breaker that is not used where backpressure is expected. A low hazard may use those or a double check valve assembly. Table 1 is a guide. The policy says each service is evaluated on its own. Shops named on that table are discussed on the high-hazard commercial page. Deadlines for the test form are on the Anderson, Pickens, and Oconee page.

The policy’s installation section for this assembly says:

• RP assemblies are above ground unless Electric City Utilities specifies otherwise.
• The relief opening points toward the ground. The policy allows no vertical or angled installation.
• An RP inside a building or in a pit needs a floor drain by gravity, sized at least twice the relief-valve diameter. A sump pump is not a substitute.
• The assembly is not placed in a flood plain or where the relief valve can be submerged.
• The relief valve is not piped solid to a drain. The air gap is at least twice the opening size and never less than 2 inches.

The policy says Electric City Utilities recognizes only the differential-pressure method for an RP assembly, using a three-hose or five-hose differential gauge. If an RP is in a pit, the tester inspects the pit drain at the annual test, writes that inspection on the form, and a drain that is slow or clogged is cleaned or repaired immediately.

Table 2 in that policy is Anderson’s minimum for a passing test. It is not printed in the Greenville Water or Spartanburg Water sections read for this page, and it is not a statewide number from R.61-58.7(F). For an RP assembly, Table 2 lists:

• Check number 1: 5.0 psi minimum and 11.0 psi maximum
• Check number 2: 1.0 psi minimum and 4.0 psi maximum
• Relief valve: 2.0 psi minimum and 5.0 psi maximum

The policy still says “SC DHEC” for the certifying agency. Current tester cards are SCDES cards.

What the report contains

The state rule, section (7), says a certified tester tests a reduced pressure principle assembly after installation and before the customer uses it, at least once a year, and again immediately after any repair. The water system gets a written report. The tester who did the work submits it. What is backflow testing is the longer account of that section.

The SCDES sample form records, for a reduced pressure backflow preventer, check valve 1, check valve 2, and the relief valve, with an initial test and a final test, a line pressure, and a pass or fail line. Spartanburg Water’s field test report has a checkbox for RP and the same three results, before repair and after repair. Greenville Water and Spartanburg Water, in the manual sections read on October 7, 2026, do not print the psi bands Anderson prints. The pass or fail on those systems is the result the certified tester enters on the form those utilities accept.

Who can do the test

In R.61-58, a certified tester holds an up-to-date card from the Department. The classification that tests for the general public is the General Tester. A Limited Tester tests only assemblies the employer owns. Renewing the card includes a wet-lab test of a reduced pressure principle assembly on the tester’s own differential gauge. General vs Limited has the course and the gauge accuracy the regulation states.

County pages copy General Testers from the SCDES report printed July 9, 2026. Start with Greenville County, or the county on the tester’s address. Confirm the current card before you hire. A row on this site is not an appointment.

The regulation does not set a price for the test. The cost guide keeps a third-party range separate from any utility fee. Official links are on Resources.

Sources read October 7, 2026: R.61-58 on des.sc.gov, the SCDES program page, the SCDES sample test form, the January 1, 2026 approved-assembly list, Greenville Water’s backflow page and November 10, 2025 manual, Spartanburg Water’s cross-connection page, June 4, 2026 manual, and field test report, and Anderson Electric City’s September 2008 policy.