06 Bijzondere werkzaamheden
 >  Confined Spaces

So-called ‘confined spaces’ that need to be entered to perform certain work may be present at Heijmans construction sites and workplaces.

Prior to entering a confined space, the risks must be assessed and suitable mitigating measures must be taken. This toolbox deals with commonly occurring risks and the associated mitigating measures.


What is a confined space?

A confined spaces is a closed or partially open space, with or without a narrow access point. Such spaces are not designed for people to stay in, generally provide limited room for movement and have limited access and escape routes. Furthermore, potentially they may contain residues of toxic or flammable substances, which can result in lower-than-acceptable oxygen levels.

Examples of confined spaces:

Tanks, cooling towers, buffer vessels, cellars, cable and other crawlspaces, sewers and manholes, elevator shafts, double walls/bottoms, excavated wells/trenches deeper than

1.80 m, air conditioning cabinets, pipeline tunnels, steam boilers, water basins, sprinkler/rainwater wells.


The workplace in a crawlspace is difficult to enter and ventilate. They are often dark, tight and difficult to quickly escape from in case of emergency.

The desired dimensions of a crawlspace are:

  • Crawl hole at least 62x100 cm
  • Crawlspace height at least 80 cm
  • Clearance (below beams and pipelines) 60 cm
  • The maximum distances to a crawl hole from the crawlspace are 7.5 metres at a height of less than 80 cm and 18 metres at a height of 80 cm or more.

The maximum working time in this crawlspace is 1.5 hours followed by a 15-minute break.

In case the dimensions of the crawlspace are smaller, the maximum working time in this space is 1 hour followed by a 15-minute break.


General requirements

For everyone involved in entering or working in a confined space, all of the following conditions apply before work may be started:

  • A work permit has been requested whereby the risks and mitigating measures are set out in a Task Risk Analysis (TRA);
  • The work permit at a minimum covers the risks identified in the RI&E, supplemented by the workplace and its specific risks;
  • The way in which the work is to be carried out has been discussed with the project’s Safety Coordinator prior to applying for a work permit, preferably with an on-site assessment of the situation;
  • A Last-Minute Risk Analysis (LMRA) has been carried out;
  • A manhole watch has been requested to supervise the confined space while the work is being carried out.

Manhole watch & Gas measurement specialist

Manhole watch & Gas measurement specialist

The manhole watch:

  • Is the person responsible for communicating with and registering the persons in the confined space and is at all times present when work is being performed inside the confined space;
  • Must be familiar with the risks, the content of the work permit and the TRA, as well as the risk mitigating measures;
  • Must be capable of immediately calling for help without leaving the manhole;
  • Must be able to implement emergency measures, with the exception of entering the confined space, until emergency assistance arrives.

A manhole watch may NEVER:

- Collaborate in performing the work, since this can distract him/her from the manhole watch task;

- Let the manhole out of sight as long as there are persons present in the confined space or the confined space has not been left safely secured.

Gas measurement specialist:

A gas measurement may only be performed by a qualified gas measurement specialist. This person must possess the right qualifications and a valid gas measurement certificate.


Risks > What can happen?

A dangerous atmosphere may be present in confined spaces or can result from the work being performed. The hazards this can create include suffocation, intoxication, poisoning, fire and explosion.

  1. Suffocation: is when the body can no longer perform its breathing function. In confined spaces, workers can, for example, suffocate because inert gases can cause oxygen to be displaced.
  2. Intoxication: consists of losing or being in a state of reduced consciousness due to exposure to a certain substance.
  3. Poisoning: also referred to as intoxication, is the ingestion of a certain quantity of a substance, which causes your body to become damaged.
  4. Fire: is an undesirable combustion by fire that can spread unimpeded and cause damage and/or danger.
  5. Explosion: is a sudden enlargement of the volume of a quantity of material and the release of energy in a violent way, usually in association with the creation of high temperatures and the release of gases.
Measures > for working in confined spaces

Various measures must be taken before the confined space may be entered and before the work may be started, as follows:

  • The access points to this space must be provided with a prohibition sign that clearly displays the text ‘Danger! Confined space/Access prohibited to unauthorized persons’;
  • Disconnect or blind pipelines as closely as possible to the confined space, to prevent gases or liquids from entering the confined space (Lock-out Tag-out Try-out work instruction);
  • Have a gas measurement specialist perform gas and/or oxygen measurements;
  • Moving parts in a confined space must be switched off in such a way that they cannot be put back into operation easily;
  • Safe voltage must be used in confined spaces (50 V alternating current or 120 V direct current).
                                         Illustration: release measurement by a gas measurement specialist

 The following measures must be implemented while working in a confined space:

Fence off the area to keep out persons who do not have any tasks near/in the confined space;

  • Keep access and escape routes free from any obstructions in relation to any emergency situations;
  • A personal gas meter must be used;
  • If there is no certainty that the confined space continues to be safe for humans, measurements must be frequently and possible continuously taken while the work is being performed;
  • Keep suitable fire extinguishing equipment available and ready for use;
  • Bottles containing gases (such as oxygen) may never be put in confined spaces. During work stoppages, the burner as well as the bottles must be shut off and removed from the confined space. 
                                             Illustration: Multigas meter for continuous gas measurement
Discussion > Discuss this with your colleagues!

Discuss the following topics with each other:

  1. Why is due caution for entering and working in confined space really necessary?
  2. Am I aware of all the risks that can occur in the confined space where I will be working and am I taking all applicable measures?
Tips > For more information

Heijmans Academy Training: Conducting gas measurements for working in confined spaces

Volandis Knowledge Centre: Working in confined spaces

Aboma Consultancy: Abomafoon 6.01 Working in confined spaces

MicroRAE portable gas meters method of operation instruction video

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