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Confined Space Water Tank Work | Sharing In-house Equipment and Supplier Verification Points
2026.10.13 13:42
Serious Accidents and Occupational Safety and Health Act
Confined Space Water Tank Work | Sharing In-house Equipment and Supplier Verification Points
Even if you entrust the water tank cleaning to a company, there is a risk of suffocation inside the tank. enclosed space This may be the case. Measurement, ventilation, monitoring, and protective equipment are directly performed by the contractor employing the workers, and the company managing the building Provide risk information in writing and verify implementation We have divided the tasks into a table: who prepares the equipment and what to check.
short answer
Even for water tank cleaning performed only once every six months, if the work involves workers entering a confined space, oxygen and hazardous gas measurements, ventilation, placement of supervisors, protective equipment, and special training are all required. This obligation rests with the employer who hired the workers. Subcontractor (water tank cleaning contractor) Performed as the business owner, and entrusted the cleaning Contractor It must provide hazard information in writing before work begins (Article 65 of the Occupational Safety and Health Act) and verify that measures have been taken in accordance with those instructions. While the company is not obligated to purchase all the equipment, it must provide measuring equipment and protective gear if in-house employees are entering the area directly. The key point is not so much 'who brings the equipment' Who checked what and kept records on the day of the work It is to divide it in advance.
Even for water tank cleaning performed only once every six months, if the work involves workers entering a confined space, oxygen and hazardous gas measurements, ventilation, placement of supervisors, protective equipment, and special training are all required. This obligation rests with the employer who hired the workers. Subcontractor (water tank cleaning contractor) Performed as the business owner, and entrusted the cleaning Contractor It must provide hazard information in writing before work begins (Article 65 of the Occupational Safety and Health Act) and verify that measures have been taken in accordance with those instructions. While the company is not obligated to purchase all the equipment, it must provide measuring equipment and protective gear if in-house employees are entering the area directly. The key point is not so much 'who brings the equipment' Who checked what and kept records on the day of the work It is to divide it in advance.
📌 Why a water tank becomes a sealed space
Article 618 of the "Rules on Industrial Safety and Health Standards" defines a confined space as "a place where there is a risk of oxygen deficiency, suffocation, fire, explosion, etc. due to hazardous gases, as specified in Annex 18." Although the word "water tank" does not appear directly in Annex 18, it includes the interiors of tanks, manholes, and pits, and the interiors of steel tanks that have been sealed for a long period of time. The interior of a place where access is restricted as a space where workers do not reside. Since this includes [etc.], the interior of a building's water tank may be subject to this. The Ministry of Employment and Labor also cited work inside water tanks and septic tanks as examples of work posing a risk of suffocation in a press release regarding the revision of the regulations on confined spaces in December 2025.
Since it is a space normally filled with water and inaccessible to people, the internal air conditions cannot be determined from the outside immediately after draining the water. Therefore, rather than the familiar job name 'water tank cleaning' Confined space work It is important to classify it first and start the procedure.
| Standards for adequate air quality (Article 618) | Reference value | If you get away |
|---|---|---|
| oxygen | 18% or more and less than 23.5% | If it is less than 18%, there is an oxygen deficiency |
| carbon dioxide | Less than 1.5% | Re-measure after ventilation |
| carbon monoxide | Less than 30 ppm | Re-measure after ventilation |
| hydrogen sulfide | Less than 10 ppm | Re-measure after ventilation |
If the measurement results indicate that the air is not adequate, ventilation must be provided or air respirators or supplied-air masks must be worn (Article 619-2, Paragraph 3; Article 620).
🏢 Companies That Need This Article | Water Supply Act Reservoir Management Targets
Article 33 of the Waterworks Act and Article 50 of the Enforcement Decree of the same Act impose an obligation on owners and managers of buildings of a certain size or larger, such as buildings with a gross floor area of 5,000 m² or more, office facilities with a gross floor area of 3,000 m² or more, large-scale retail stores, and apartments, to manage the hygiene of water storage tanks. The main obligations pursuant to Article 22-4 of the Enforcement Rules of the Waterworks Act are as follows.
- Water tank cleaning At least once every six months , hygiene inspection At least once a month
- Water quality test At least once a year (Requested to a designated drinking water quality testing agency)
- After cleaning, fill with water and check that the residual chlorine is 0.1–4.0 mg/L, pH is 5.8–8.5, and turbidity is 0.5 NTU or less.
- Record of cleaning, inspection, and water quality test results Store for 2 years (Article 22-6)
Cleaning and hygiene inspections reported in accordance with Article 34 of the Water Supply Act Water tank cleaning contractor The task may be delegated to another party, and when doing so, a cleaning supervisor must be stationed at the site (Article 22-5 of the Enforcement Rules). In other words, since the Water Supply Act stipulates the 'duty to manage water cleanly' and the Occupational Safety and Health Act stipulates the 'duty to protect those entering the area,' both must be taken care of together. This is a task primarily handled by general affairs, facility, and safety managers at companies that directly manage buildings, such as headquarters buildings, factory dormitories, and logistics centers.
⚖️ Legal Roles | What Do the Contractor and Subcontractor Do?
If the cleaning of the water tank is entrusted to a company, the company becomes the client and the cleaning company becomes the contractor. Work in confined spaces falls under "work performed in places where there is a risk of suffocation" as stipulated in Article 54 of the Enforcement Decree of the Industrial Safety and Health Act, and therefore, pursuant to Article 65 of the Act Provision of safety and health information documents This is the subject. Since there is no separate table of role division in the statutes, the classification below is organized based on the articles.
| division | Contractor (Building Management Company) | Subcontractor (water tank cleaning contractor) |
|---|---|---|
| Information provision | Risk information, precautions, and accident measures before starting work to document Provided (Article 65(1) of the Act, Article 83 of the Enforcement Rules) | If information is not received, request it. If it is not provided even after the request, the work may not be performed (Article 65, Paragraphs 2 and 4 of the Act). |
| Confirmation of implementation | Verify whether the contractor took action in accordance with the information (Article 65, Paragraph 3 of the Act) | Share the results of measures such as measurements, ventilation, and protective equipment with the client. |
| Workplace management | Access control, coordination of power and water supply cutoff, coordination of mixed work,巡回 inspection (Articles 63 and 64 of the Act) | Control of access to work areas, posting of pre-work checklist at entrances (Rule 619, Paragraph 3) |
| Measurement and ventilation | There is no direct obligation if internal employees do not enter. Securing copies of measurement records is recommended. | Designation of measurers and provision of equipment, measurement before and upon resumption of work, preservation of result records for 3 years (Article 619-2), ventilation (Article 620) |
| surveillance and rescue | Sharing emergency contact systems, guidance on 119 reporting routes | Stationing of monitors and constant communication facilities (Article 623), provision of evacuation equipment (Article 625), emergency rescue drills once every six months (Article 640) |
| education | Provision of training venues and materials, and verification of implementation (Article 64 of the Act) | Special training for confined spaces (Tables 4 and 5 of the Enforcement Rules), notification of risks and emergency contact information before work (Article 641) |
The measurement standard has changed. Prior to the amendment on June 28, 2024, there was a list of persons capable of measurement, such as supervisors and safety managers, but the current Article 619-2 states that the employer Person with knowledge and practical experience in measurement and evaluation It is stipulated that [it] be designated and measuring equipment be provided. With the amendment effective December 1, 2025, there is also an obligation to record the name of the measurer, the date, time, location, and results, and to preserve them for three years. Please be careful not to use the list of previous data as is.
🧰 Dividing preparation responsibilities by equipment
The company does not need to be fully equipped for semi-annual operations. However, the legal responsibility to provide the equipment is The employer of the worker using that equipment Since this is the case, you can reduce confusion on the day by determining the preparation party and verification method as follows during the contract stage.
| equipment | Related provisions | Preparatory entity (in general cases) | Things the contractor must verify |
|---|---|---|---|
| Oxygen and hazardous gas detector | Article 619-2 | supplier | Calibration/inspection status, name of the measurer, copy of measurement record |
| ventilation fans and ducts | Article 620 | Subcontractors (all in cooperation with the client) | Installation location, ventilation time before work |
| air respirator or supplied-air mask | Article 619-2③ · Article 620 · Article 624 · Article 625 | supplier | Emergency quantity, wearable personnel |
| safety harness, lifeline, tripod (winch) | Article 624 | supplier | Structure method suitable for manhole depth |
| evacuation equipment such as ladders and fiber ropes | Article 625 | supplier | Whether it is placed near the entrance |
| Constant communication equipment (walkie-talkies, etc.) | Article 623③ | supplier | Whether communication is possible in the underground machine room |
| No Entry Sign | Article 622 | Contractor (normal) · Subcontractor (during work) | Normal manhole lock and sign |
Would it be okay to handle this by renting in-house equipment?
If in-house employees are responsible for internal inspections of water tanks or emergency response, the company, as the employer, must provide the equipment. In this case, since there is no regulation mandating purchase, renting is also an option. The Korea Occupational Safety and Health Agency One-Call Service for On-Site Suffocation Prevention We provide free rental of multi-gas concentration meters, ventilation fans, power-supplied air masks, tripods, etc., and support training on measurement and usage (☎ 1644-8595). However, positive-pressure air respirators are not available for rental, and the Corporation advises that concentration measurements taken by implementing agencies do not substitute for the workplace's legal assessment of adequate air quality. Even with rented equipment, the company must directly designate the measurer and record the measurements, and KOSHA GUIDE H-80-2021 recommends calibrating the reference concentration and alarm setting concentration before measurement.
🧭 5 Steps to Cleaning a Water Tank in a Confined Space
1) Contract Phase | Document vendor qualifications and roles
The contract or work plan must specify whether the water tank cleaning business has been registered, the placement of cleaning supervisors, the entity responsible for equipment preparation, the measurer, the monitor, and the method for submitting proof of special training completion. Work in confined spaces is subject to special training as specified in Annex 5 of the Enforcement Rules, and the training duration is, in principle, at least 16 hours (at least 2 hours for short-term/intermittent work and daily workers, etc.). The scope of verification for subcontractor training completion is Related Inquiry Please also refer to this.
2) 1 week prior to work | Risk information is provided in writing.
The location of the water tank, manhole size, depth, and internal structure, recent water quality and inspection records, cleaning and disinfection chemical usage plans, methods for shutting off power and water supply, and the contact sequence in case of an accident are delivered in writing. The method for recording delivery and receipt confirmation separately is as follows. Confirmation Checklist for Provision and Receipt of Safety and Health Information Before Contract Work It is organized in.
3) Before starting the work on the day | Check and post the 6 items
Article 619, Paragraph 2 stipulates that before the start of work, the following must be verified: ① the date, time, duration, location, and content of the work; ② information on workers, such as supervisors, employees, and observers; ③ results of oxygen and hazardous gas measurements and follow-up measures; ④ review of the possibility of hazardous gas leakage, inflow, or occurrence and follow-up measures; ⑤ protective equipment to be worn; and ⑥ the emergency contact system; and that this information must be maintained until the work is completed. Posted at the entrance Please ensure that you do so. If the contractor keeps photos or copies of the posted content and measurement results, they serve as grounds for verifying performance.
4) During operation | Verify re-measurement with the supervisor
A supervisor is stationed outside the confined space at all times, and if any abnormalities occur, he/she reports to the competent fire department (119) without delay and notifies the supervisor (Article 623, amended on Dec. 1, 2025). The number of personnel is checked every time there is entry (Article 621), and is measured again when re-entering after work has been stopped for a break, etc. (Article 619-2①). Second accident of going in barehanded to rescue To prevent this, please share 'call 119 without entering' with both the supervisor and surrounding staff in advance.
5) After the work | Keep two records separately.
Records under the Water Supply Act (cleaning date, verification of residual chlorine, pH, and turbidity, company name, and retention for 2 years) and records under the Occupational Safety and Health Act (preservation of measurer and measurement results for 3 years, pre-work checks, and training records) differ in their legal basis and retention periods. Since the Facilities Team and Safety Team keep records separately, often resulting in one side being missing, it is recommended to group them together in one folder.
✅ Water Tank Cleaning Contractor Pre-inspection Checklist
Below is an internal inspection checklist for companies outsourcing water tank cleaning to verify before the work date. In particular Provision of risk information documents, designation of measurers and monitors, emergency contact system These are basic items. If three or more are lacking, it is recommended to align the work plan with the contractor before finalizing the work date.
| Inspection items | Confirmation details | check |
|---|---|---|
| Closed space registration | A water tank is included in the list of confined spaces at the workplace, and there are normally manhole lock and no-entry signs. | □ |
| Company qualifications | We checked whether the water tank cleaning business was registered and the plan to deploy cleaning supervisors. | □ |
| Information provision | Risk information, precautions, and measures to be taken in case of an accident were provided in writing, and confirmation of receipt was received. | □ |
| Equipment sharing | The entity responsible for preparing measuring instruments, ventilation fans, air respirators or air-supplied masks, and rescue equipment was determined. | □ |
| rater | I checked the name of the measurer and the calibration and inspection status of the measuring instrument. | □ |
| guardian | We established a method for constant contact with external monitors (including underground communication status). | □ |
| education | Proof of completion of special confined space training for deployed personnel was obtained. | □ |
| Emergency contact | The 119 call center operator, building entry and exit routes, and ambulance entry location were shared. | □ |
| record | The location for storing the Waterworks Act cleaning records (2 years) and measurement and verification records (3 years) was determined. | □ |
🤝 Mental Recovery and EAP After Accidents & Near Misses
Suffocation accidents can be life-threatening in a short period of time and leave a deep trauma on colleagues at the scene or employees who attempted a rescue. Even if it is not an actual accident, the experience of hastily evacuating after a detector alarm went off can lead to anxiety about the next task.
In such cases, separate from accident investigation and measures to prevent recurrence, on-site staff and witnesses EAP psychological counseling It is helpful to guide them on how to make a choice. Contractor employees should first check their company's support system, and manage whether or not counseling was used, as well as the content thereof, separately from accident investigation data. In emergency situations, calling 119 and taking first aid measures take precedence over EAP, and EAP does not replace medical treatment or official investigation procedures.
❓ FAQ
Q1. Is a confined space work program necessary even though the work is done only once every six months?
It is necessary. Article 619 stipulates that employers must establish and implement a program when workers perform tasks in confined spaces, regardless of work frequency. If only the company's own employees enter the space, the obligation to establish the program lies with the company; the company only needs to focus on managing location and access control by including water tanks in the list of confined spaces and providing information prior to work.
Q2. Do I just need to keep the measurement results from the company?
If no in-house employees enter, the obligation to take measurements lies with the company that hired the workers. Since the company must verify, in accordance with Article 65, Paragraph 3 of the Act, that measures were taken as per the information provided by the company, it is recommended to obtain copies or photographs of records containing the name of the measurer, the time of measurement, and the measured values. If the measured values exceed the appropriate air quality standards and there is no record of re-measurement after ventilation, please verify this before resuming work.
Q3. Who can perform the measurement?
The current Article 619-2 does not require a list of qualifications but mandates that employers designate persons with knowledge and practical experience regarding measurement and evaluation. Prior to work, employers must verify that the designated measurer understands the risks of confined spaces, inspection and operation methods for measuring equipment, measurement methods, and appropriate air standards, and provide necessary training.
Q4. Is it okay for a company employee to enter for the rescue?
Entering without protective equipment is extremely dangerous. Regulations have been changed requiring supervisors to report to 119 without delay if any abnormalities occur, and rescue operations must be conducted only while wearing an air-convection device or a supplied-air mask. Please also inform company employees in advance of the "do not enter, report" principle.
Recommended Posts
Checklist for Preparing Confined Space Work Before Water Tank Cleaning | Excel for Equipment Allocation and Gas Measurement Records → Confirmation Checklist for Provision and Receipt of Safety and Health Information Before Contract Work → Record standards for reporting and approving the resumption of work suspension requests at contract and service sites → Serious Accidents Punishment Act Response Checklist and Safety and Health Management System Guide →
👉 Next step
Accidents that occur in a split second, such as suffocation or falls, leave a lasting impression on the employees present at the scene. Please prepare not only for safety management but also for the emotional recovery of your employees following an accident.
Inquire about psychological support for field staff →📚 Source and Verification Criteria
- National Law Information Center · Rule 618 (Definitions) · Appendix 18 (Confined Spaces)
- National Law Information Center · Rules on Occupational Safety and Health Standards Articles 619, 619-2, 620–625, 640, and 641
- National Law Information Center · Articles 63, 64, and 65 of the Occupational Safety and Health Act (Provision of Information on Safety and Health by the Contractor, etc.)
- National Law Information Center · Article 54 of the Enforcement Decree of the Occupational Safety and Health Act (Work with risk of suffocation or collapse)
- National Law Information Center · Articles 83 and 85 of the Enforcement Rules of the Occupational Safety and Health Act, Annex 4 and Annex 5 (Special Education)
- National Law Information Center · Articles 33 and 34 of the Water Supply Act, Articles 22-4 to 22-6 of the Enforcement Rules of the Water Supply Act (Cleaning, Inspection, and Recording of Water Reservoirs)
- Ministry of Employment and Labor · Amendment and Promulgation of the Rules on Industrial Safety and Health Standards to Strengthen Prevention of Asphyxiation Accidents in Confined Spaces (December 1, 2025)
- Korea Occupational Safety and Health Agency · On-site Asphyxiation Accident Prevention One-Call Service (Equipment Rental)
- Korea Occupational Safety and Health Agency · KOSHA GUIDE H-80-2021 Technical Guidelines on the Establishment and Implementation of Confined Space Work Programs
This article is a general practical guide summarizing the preparations required for confined space work when contracting out building water tank cleaning. Whether a specific tank qualifies as a confined space and the scope of specific obligations for the client and contractor may vary depending on working conditions and contract details. The table regarding the responsible parties for equipment preparation is a practical recommendation based on relevant legal provisions. For matters requiring site-specific judgment, please verify with the competent regional employment and labor office or the Korea Occupational Safety and Health Agency. Date of verification of laws and official data: October 13, 2026.
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