Dealing with Dead Shorts
Dealing with Dead Shorts in the Field
If you followed our previous guide, How to Identify Electrical Leaks, and found that the problem is in the Wireline then you will need to determine where the leak is relative to the tool and determine if it will be possible to repair in the field or not. Although there are several ways to identify electrical leaks in the wireline, there is only one method that can be used effectively in the field. Regardless of the measurement technique it is critical that the length of the cable is known accurately. This is one of the reasons it is imperative that accurate cable record books be maintained.
The only leak type that can be effectively dealt with in the field is a dead short. To determine if you have a dead short, you will need a good quality ohmmeter (Fluke) that reads to a minimum of 0.1 Ohms. Using the methodology below you can test to see if you have a dead short.
- First short your meter leads and record this resistance reading. To eliminate errors due to the resistance of the leads, this lead resistance should be subtracted from all resistance measurements.
- Disconnect both ends of the cable and ensure the leads are clean.
- Measure the resistance of the conductor to ground (armor) on the whip end Rw
- Then reverse the leads and again measure Rw. If the resistance is significantly different with the leads switched then you have a wet leak and precise location in the field is not possible. Proceed to step 7. If the readings are the same proceed to step 5.
- Measure the resistance of the conductor to ground (armor) on the drum end Rd.
- If the leak is not wet, use the formula below to determine if you have a dead short. Where Rc is the resistance of the current length of the conductor (from the cable record book). If the value of Rc in the cable record book is questionable then measure the conductor resistance. If (Rd + Rw – Rc) < 300 Ω then you have a dead short otherwise you have a high resistance leak. If you have a dead short skip section 7 below.
- If there is a high resistance or wet leak the next step before going to a service center is to unreal several hundred feet of cable and with the Ohm meter attached at the drum end watch the Ohm meter as this loose cable is flexed along the length by hand. This is a crude but sometimes very effective way of leak location as many cable leaks occur within a few hundred feet of the whip end. If the leak is found cut the cable and test. If the leak is still present, the cable must be taken to a service center.
If the resistance is less than 300 Ω than follow these steps to determine the location of the leak.
- Find the total length (L) of the cable from the cable record book
- Calculate the length of cable to the leak from the whip end (Lw) using the following formula.
- Lw = (Rc + Rw – Rd) x (L / (2 x Rc))
- Calculate the length of cable to the leak from the drum end (Ld) using the following formula.
- Ld = (Rc + Rd – Rw) x (L / (2 x Rc))
The example below demonstrates how to use the formulae:
- L = 24,500 ft (7,468 m) For metric use meters in the same formulae
- Rc = 68.6 Ω
- Rw = 149.4 Ω
- Rd = 205.2 Ω
First test to see if it is considered a dead short (Rd + Rw – Rc) = 286.0 Ω Less than 300 Ω
Check leak distance from whip end Lw = (68.6 + 149.4 – 205.2) x (24,500 / (2 x 68.6)) = 2,286 ft
Check leak distance from drum end Ld = (68.6 + 205.2 – 149.4) x (24,500 / (2 x 68.6)) = 22,214 ft
Once you believe you have located the leak, slack the cable and examine it for physical damage and manually flex the cable while observing the leakage resistance. You should find variations in the ohmmeter reading if you are at the leak. If the leak is located close enough to the whip end that you can spool down and cut off, and you have enough cable remaining to perform the current services then do so. After the short is cut out, test the remaining cable to ensure electrical integrity. Rehead and finish the job.
If the leak is too far away from the whip end to cut out then you will have to take the cable into a service center for inspection. At a service center they are well equipped to deal with all types of leaks. Burning a leak out with a “burn out” box is the fastest way to identify difficult leaks. The service center will also have more sophisticated equipment than an ohm meter to try and locate the exact point of failure. If it is suspected the cable is damaged from mechanical damage or temperature, then a burn out box will save time to locate the leak. However, if it is important to determine the cause of the leak, then a burn out box should not be used as it will destroy the area around the leak and likely make it impossible to perform a root cause analysis of the failure. Note as in the picture to the right, damage to both conductor and the armor can result from use of a burn out box.