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As pointed out in this month’s companion article “Safety Tips”, there is always the potential for atmospheric hazards in manholes. To protect yourself from these unseen but potentially deadly hazards, always adhere to the safety practices established for your industry. The safety practices followed by the telecommunications industry and power companies are somewhat different when it comes to manhole atmospheric testing and monitoring, but the underlying objectives are the same, to ensure the safety of all personnel working in manholes.

The OSHA Standard for Telecommunications is 1910.268. Under the sections of this Standard applicable to manholes, most telecommunications companies now follow what is commonly referred to as the “Test, Purge, & Ventilate” routine. This practice provides for the use of a combustible gas detector or combustible gas monitor to first determine if there are combustible gases present prior to entry. The use of a combustible gas detector or monitor is then coupled with the use of a power ventilator to ensure a safe atmosphere within the manhole before entry, as well as while personnel are inside.

NOTE: If initial testing also includes testing for oxygen levels, ventilation does not have to be provided unless the initial testing showed an oxygen deficient atmosphere or other hazard. Although testing oxygen levels is not required by this Standard, some of the “Telco’s” are now using, or exploring the use of multi-gas units to test and monitor for both combustible gas and oxygen levels.

Electric Power Generation, Transmission, and Distribution
The OSHA Standard for the Electric Power Generation, Transmission, and Distribution is 1910.269. Under the sections of this Standard applicable to manholes, certain safety procedures need to be in place. For example, before an employee enters an enclosed space such as a manhole, the internal atmosphere must be tested first for oxygen deficiency, then for the presence of combustible or flammable gases. If continuous forced air ventilation is provided, testing is not required as long as the procedures used will ensure that employees are not exposed to the hazards posed by oxygen deficiency or flammable gases. If flammable gases or vapors are detected initially, or if an oxygen deficiency is found, forced air ventilation must be used to maintain oxygen at a safe level and to prevent a hazardous concentration of flammable gases and vapors from accumulating. A continuous monitoring program to ensure that no increase in flammable gas or vapor concentration occurs may be followed in lieu of ventilation, if flammable gases or vapors are initially detected at safe levels.

Whether you are working for the telecommunications industry, or for a power company, there could be unseen environmental hazards in any manhole. Be sure that you always follow the safe work practices established by your industry in concert with OSHA to ensure not only your own safety, but the safety of your fellow workers as well.

Whatever industry you work in, U-TECK has the equipment you need to ensure your safety while working in manholes. For example, we have the “Protector” for multi-gas detection applications, including oxygen, and our “Gas Trac NGX-6” and Multi-RAM RDA IIIA have proven themselves over the years for single gas combustible gas testing or monitoring. Any of these units can be used by either telecommunications or power company personnel in conjunction with your industry or company manhole safety practices.

Multi-Ram RDA III A


Protector AR

The following is an outline of the “Test, Purge and Ventilate” routine referenced earlier in this document. Although this manhole safety procedure is utilized primarily by the telecommunications industry, there are portions that certainly could be utilized by other work groups, including power companies or sewer workers. For that reason we are sharing it with our web site visitors.

Testing, Purging & Ventilating Manholes
Step 1

Perform an initial Test prior to entry for combustible gas:

NOTE: If it is your company or industry practice to also test for oxygen, test first for oxygen

• Whenever possible, before removing the manhole cover, Test through the puller access holes at the top of the manhole “chimney”

NOTE: Any of the U-TECK Gas Detectors such as the “Protector” or “Gas Trac” and Continuous Gas Monitors such as the Multi-RAM RDA IIIA and the NGX-6-CMR can easily be utilized in this manner with our accessory “30-inch non-conductive probe”.

• Immediately after removing the manhole cover, test all levels inside of the manhole

Step 2

Purge with a power ventilator

What is purging?
Purging is the provisioning of fresh air from above ground into the manhole via a power ventilator (blower). Purging the atmosphere within a manhole before entering is important for three reasons:

• To purge out or expel any combustible gases.
• To purge out or expel any hazardous vapors.
• To replace any “dead air” (oxygen deficient) atmosphere with fresh air.

How long should I purge a manhole before entering?
Purge times are based on the following primary factors:

• The size of the manhole or other space to be purged.
• The airflow rate of the blower being utilized, normally expressed as Cubic Foot Per Minute (CFM).
• The length and diameter of the blower hose.
• Whether or not the initial test of the manhole atmosphere indicated the presence of combustible gas.

Reference charts for calculating normal purge times (when initial atmospheric testing indicates no combustible gas above 10% LEL) may be available within your company safety practices. These charts are based on the above factors to assist you in calculating purge times.

If no reference charts are available, a safe rule of thumb for purging the average single chimney manhole (100 cubic feet or less) using a minimum of a 500 CFM blower and a blower hose 25 feet or less with one 90-degree bend is:

• 15 minutes if the initial test indicates no dangerous levels of combustible gas (below 10% LEL).

Double the purge time if initial testing does showed levels of gas at 10% LEL or above.

Our lightweight UT-VAF-1500B blower (pictured below) is ideal for this application.

Larger manholes would require additional purge time, or the use of a higher capacity blower such as our UT-VAF-3000 (pictured below).

What other safety considerations should I be aware of while purging?

To maximize CFM airflow during purging, there should be no more than one 90-degree bend in the blower hose.

Place the end of the blower hose straight down into the manhole as far as possible. Try to get it at least 2 feet below the chimney. The placement of the blower hose in this manner maximizes the efficiency  of the purge.

Always make sure you have plenty of free air circulation around your blower, and do not place the intake side of your blower next to any sources of carbon monoxide such as:

• The exhaust from your vehicle, if it is running.
• The exhaust from a portable or built in generator.
• Heavy vehicular traffic areas
• NOTE: If impossible not to set-up away from heavy vehicular traffic, an ideal solution would be to utilize our “High Five.

Place the blower intake at least 5 feet away and upwind from the manhole opening to reduce the possibility of re-cycling into the manhole any gases you are purging out of the manhole.

The illustration below depicts a typical portable ventilator and hose set-up for purging.

Step 3

Re –Test from topside immediately after purging to ensure that the atmosphere inside of the manhole is safe to enter. If your tests indicate no unsafe conditions, you can now enter the manhole.

Step 4

Immediately after entering the manhole, test all corners and any open ducts inside of the manhole immediately after entry for pockets of gas that might not have been fully purged out.

NOTE: The 30-inch non-conductive probe described and pictured previously is ideal for this application
If you do find any pockets of gas just take the end of your blower hose, and aim it at that location(s) for a couple of minutes to try and disperse the gas. Then, re-test to ensure that the gas is gone. If you still have gas, you should exit the manhole and follow your company practices for management notification, etc.

Step 5

If you are satisfied that there is no gas, continue to Ventilate continuously with a power ventilator while anyone is working inside of a manhole.

NOTE: When in the Ventilate mode, it is permissible to add another 90-degree bend in the blower hose to facilitate circulation inside of the manhole by aiming the end of the blower hose at a sidewall.

This ensures that:

• There is a continuing supply of fresh air
• There is no buildup of combustible or toxic gases

Step 6

If using a non-continuous gas detector such as the “Gas Trac”, Test periodically per
company practice at head level, at all corners, and at any open ducts for combustible gases that might
have built up from unexpected events such as:

• Ruptured gas mains
• leaking gasoline storage tanks

If using a continuous gas monitor such as the “Protector”, “Multi Ram RDA III A” or “GasTrac NGX-6-CMR”, ensure that the sensing unit is placed within the manhole at your head level while working.

What are some additional safety precautions I should follow when working in or around manholes?
Never use the following items inside of a manhole or within 10 feet of an open manhole:

• Matches, lighters, or other devices capable of producing a spark or flame
• Lighted cigarettes, cigars or pipes
• Heaters or furnaces

Ensure that heating and lighting equipment is approved for manhole use, is “intrinsically safe” (i.e. spark proof), and that it is used properly (i.e. never plug or unplug electrical equipment inside of a manhole.

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