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What is DIT?
DIT-Duct Integrity Test is a complete series of tests performed on the underground duct for laying of optical fiber. It is the intermediate step between duct laying and before cable jetting inside the duct. Duct Integrity Test apparatus can be convienently used for testing of various parameters like duct alignment , air tightness test, shuttle test and also useful for cleaning before installations of O F C in Plb/ silicore ducts of various diameter 32mm,40mm and 50mm with safe environment.

Purpose: The main purpose of Duct Integrity Test is to ascertain and ensure the suitability of duct for optical fiber cable installation through jetting/blowing or cable pulling.

Duct integrity test for FTTH
DIT Process


What is done in DIT?
1. Check continuity of the duct
2. Clean the duct of any mud,stones,water
3. Check for Kinks, Blockages, Sharp bends
4. Check for any type of leakage in the duct and rectify the faults.

The 4 Duct Integrity Tests:

Test                                    Description                                    Purpose

1.Air Blowing                 Full Discharge from Compressor.          Check for duct continuity.

2.Shuttle Blowing          Dia. 80% of duct I.D. and                       Check for kinks,dents and    
                                      length 150mm for 40/33mm duct            sharp bends.

3.Sponge Blowing         Dia. Twice the duct I.D,4 inch in            Clean the duct of
                                      length and min. density 28gms/cft.       mud,stones,water, etc 

4.Air Pressure               Allow 5 bar for 30 minutes.Permissible  Check for leakage
                                      drop consider 0.5 bar.                             and punctures.    

1. Normal Air Flow inside the duct ……… Test Passed, proceed to next test
2. No Air flow ……………..........................Test has failed.
                                                                   Reasons: a. Duct missing
                                                                  b. Duct overlap-Coupler missing
                                                                  c. Blockage (mud, plastic, etc.)
3. Low Air Pressure………Test has failed
                                          Reasons: a. Loose / faulty fitted coupler
                                          b. Puncture
                                          c. Partial blockage
4. Back Pressure…………Test has failed
                                          Reason: Blockage.

➤Commonly use a 40mm long mandrill made from Nylon or Teflon.
➤The OD (Outer Diameter) must be no more than 85% of the microduct ID
➤At the receiving end of the duct, a DIT catcher must be used for the safety purpose.
TIME for shuttle to come out at the other end or receiving end:
1 km ……. Approximately 1 - 1.5 minute
2 km ……. Approximately 3 - 3.5 minutes
If the test fails……. Blow transmitter
Reason for failure: (a). Kink or dent (b). Bending radius not maintained.

➤A sponge is generally used 100mm in length and 2 x the duct ID.
➤Wet the sponge slightly with blowing lubricant.
➤Place the tight-fitting foam sponge inside the microduct.
➤In DIT process at a pressure level of 10 bars, blow the sponge through the microduct.
Note: It is important to that Sponge could be blown before shuttle, then there is a possibility of it getting stuck. Inference same as Shuttle blowing test result.


Test parameters:
               1. Time : 30 minutes
               2. Pressure : 5-10 bar pressure
Permissible pressure drop: 0.5 bar pressure
Reason for failure of test: Leakage at coupler points, Puncture in duct.

Duct Integrity Test Faults:

1. Improper duct laying
- Bending radius not maintained while trenching
- Uneven trench bottom/ faulty trenching
- Decoiler not used while uncoiling.

2. Duct not continuous
- Missing section of the duct
- Coupler not installed.

3. Kink in the duct
- Spiraling due to non-use of decoiler
- Boulder used in back-filling the trench
- Sharp bend could be occurred in duct at 900 crossings/ culvert, etc.

4. Mud & Water in duct
- End plug not used while duct laying
- Ends not coupled/ not sealed before back-filling
- Duct laid in water-filled trench or laid on a rainy day.

5. Leakage in the duct
- Duct damaged while handling/ laying
- Duct not checked before laying
- Leakage at Couplers:
* Improper duct cut
* Improper sequencing of the coupler parts
*Coupler not sufficiently tightened.

Safety :

* Do not go inside the pit while DIT is being performed
* During the DIT do not stand in front of the ducts when:
Air/ Shuttle/ Sponge/ Transmitter is blown
* Suddenly do not open couplers or PTE before pressure is released completely
* Use warning tape for the safety with cones wherever pits are opened
* Use proper warning cone and tape around the compressor for proper identification for moving traffic.

Post a Comment


  1. Is there any methodology or possible way to explain how to identify the duct air leakage location either joints area or puncher in the duct even the sponge and mandrel test is passed but leakge in duct.

  2. A - if duct are not laid : use colored powder/water.
    B - if duct is laid : there is a duct scanner with camera you can evaluate the crack or air leaking point...

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  6. Question, if duct fails leak tests, what ramification will ensue later on.