The materials of the outer sheath of cables usually differ depending on whether the cable is laid in the ground or above the ground (in places where oxygen is available). To avoid cable fires (for example, due to a short circuit), cables for open laying are provided with a non-flammable outer sheath. However, when laying in the ground, there are no such requirements, and the cable is supplied with a conventional outer sheath. Of course, the question arises – what should be the cable sheath when laying underground, but in pipes (ducts) filled with air?

Usually, cables in pipes have a conventional flammable sheath. Also, the pipes for cables themselves often have flammable properties (for example, PE and HDPE are flammable materials). Then is there a risk of cable fire, for example, if the cable is short-circuited in a pipe section filled with air?

Yes, there is such a risk! Laboratory studies show that the risk of maintaining fire is significantly increased if the cables fill less than 35% of the internal volume in the pipe. Therefore, as shown in the photo, in practice, the ratio of the inner diameter of the pipe (Din) and the equivalent diameter of the cables (deqv) should not exceed 1.7.

In general, the inner diameter of the pipe (Din) is determined by the following factors:
1️⃣ Cable pulling conditions (forces, tensions, bending etc).
2️⃣ Cable current-carrying capacity (cable ampacity).
3️⃣ Cost of the pipe.
4️⃣ Risk of fire.

Factors 1-2 tell us that it is necessary to increase the inner diameter of the Din pipe by using more expensive pipes (Din/deqv should be at least 1.5). Factors 3-4 tell us that it is necessary to reduce the inner pipe diameter (Din/deqv should be no more than 1.7). It turns out that, when selecting the pipe, we don’t have a large diameter range, and all we have is something between 1.5 and 1.7, not less and not more.

In general, the risk of cable fire and its spread inside the pipe depends on the following two points:
✅ The volume of air in the pipe or, the filling coefficient of the pipe.
✅ Sealing of the pipe ends (especially in places where oxygen is available):
🔹 Exits from the ground to the cable terminations;
🔹 Exits to basements under the cable terminations;
🔹 Exits to manholes with cable joints;
🔹 Laying on bridges and overpasses.

Today we discussed the first point only (filling), and some other time we will discuss the second (sealing). Moreover, we will discuss the requirements to pipe materials, in terms of fire.

Let me please remind that the application of polymer materials for the cable industry is partially discussed in Part 12 of the book. Probably, It’s not something you know.

By the way, cables can ignite not only due to a short circuit, but even in normal operation, due to overheating by load currents. Here is an example of fire in normal operation.