IEEE Electrification Magazine - September 2016 - 28

Maintaining the Rail Insulation
UA

OFF

10 m

Figure 5. A measurement of rail resistance.

joints in the track area of the structure. The conductance
per length GlRS is to be measured in accordance with Figure 6 and to be calculated in accordance with (8), averaging the three measured voltage values
I - I RA - I RB
1
GlRS = $
l ^DU + DU A + DU B h 3
DU = U ON - U OFF.

(8)

Once again, the value of the current I supplied by the
source should be between 15 and 30 A. The voltage values
U ON and U OFF must be measured with the source turned
ON and OFF in a cycle, as explained earlier.
The currents I RA and I RB can be obtained indirectly
with the help of the voltage drop measurement of the
rail described in the "Testing the Longitudinal Rail Resistance" section. The voltages U A and U B are measured,
and, with the longitudinal resistance of the running rail
(RlR), it is possible to calculate the currents I RA and I RB . If
there are no points of current leakage, then the voltage
drops DU A and DU B are almost the same. A notorious
imbalance between those voltages indicates a current
leakage. To find those points, one method is to "follow
the stray current" technique by measuring the voltage
drop along the rails at each 10 m in the direction of the
bigger voltage drop ( DU A or DU B ) until the voltage drop
abruptly decays. This indicates the point where the rails
are badly insulated.

- +

U P - T+
.
U P - T+ + U P - T-

If Everything Goes Wrong
If everything goes wrong, the probability of the occurrence of stray currents will increase, and the public

Structure
-
+ I
ON OFF

URE
> 50 m

> 30 m

U
IRA

l
Figure 6. A measurement of the conductance per length for track
sections without civil structures.

(9)

and the potential diagram of the line drawn as in Figure 7.
A good method is to maintain the b factor between 0.3 and
0.7 (Figure 8). Values below 0.3 indicate a cathodic zone, and
values over 0.7 indicate an anodic zone, which is where corrosion can begin to occur.

OFF
ON

After the tests have been performed and the results are
satisfactory, the rail line is ready to be put into operation.
The most important thing from now on is the track
maintenance. The track must be regularly cleaned and
inspected to maintain the insulation levels. Measurements must be performed regularly to verify whether
there is any significant deviation from the project's original values. For a track section with civil structures, the
"follow the stray current" technique is a good method to
find leakage points. To make the maintenance work easier, the IEC 62128-2 standard also recommends continuous monitoring of the rail potential, which is registered
at dedicated locations along the line, such as in substations or passenger stations. To compensate for the varying traffic during the day, an averaging process is used.
For the averaging time, 24 h are recommended. If there is
a change in the average rail potential, a change in the
rail-to-earth conductance may have occurred, which may
be a cause for concern that there may be an increase of
stray currents. Therefore, the average value is compared
with the value of the reference situation where the system meets the standard requirements.
Another way to monitor the rail line is to set some
measuring points every 500-1,000 m and to register the
maximum positive (U p - t +) and the minimum negative voltage (U p - t -) between the tunnel body and the rails during a
time when there is maximal traffic. At each measured
point, the b factor must be calculated as in

I E E E E l e c t r i f i c ati o n M agaz ine / SEPTEMBER 2016

IRB

l
Figure 7. A measurement of the conductance per length for track
sections with civil structures.

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2016