Shielded fire-resistant cables: Why do you need both features?

Short answer: Shielded fire-resistant cable needs both features because it serves circuits that carry weak signals and must keep working during a fire. The shield keeps the signal correct during normal operation, so there are no false alarms or lost signals. Fire resistance keeps the circuit intact for 15 minutes to 2 hours during a fire, depending on the cable class. For these circuits, a cable with only one feature will fail at the other moment.

1.1 Is " shielded fire-resistant cable" a cable type or just a set of properties?

It describes properties, not a separate type of cable. "Fire-resistant" and "shielded" are two features a cable can have. What the cable is used for depends on the function of the circuit.

On the Vietnamese market, most products sold as " shielded fire-resistant cable" are fire alarm signal cables, usually with 2 cores of 1.0 or 1.5 mm². This is because fire alarm circuits are the most common application that needs both features at the same time. As a result, many people use the two names as if they meant the same thing.

1.2 Fire-resistant vs. flame-retardant cables

The two terms are often used as if they mean the same thing, but they are very different.

Fire-resistant cables are usually also flame-retardant. The reverse is not true. Learn more about what fire-resistant cables are and the standards for fire-resistant cables .

Fire resistance and shielding cannot replace each other. Each one deals with a different threat that happens at a different time

2.2 Risks of a cable that is shielded but not fire-resistant

The insulation of an ordinary shielded cable softens and burns at temperatures far below those of a fire. Once the insulation is gone, the cores touch each other or touch the screen, causing a short circuit.

The screen does not help either. Aluminium melts at about 660°C, lower than the temperatures used in fire resistance tests (830 - 950°C). So the cable section inside the fire zone loses both its insulation and its screen.

A fire protection cable route usually passes through several areas. If the route short-circuits where the fire is, devices in other areas on the same route may stop receiving commands. Evacuation speakers may go silent, smoke extraction fans may not start and fire dampers may not close, just when people in the building need them most.

2.3 When do you need only one of the two features?

Not every application needs shielded fire-resistant cable. In some cases, having both features is unnecessary. Also, the more features a cable has, the more it usually costs.

Ask two questions: does the circuit have to work during a fire, and does it carry signals that are sensitive to interference or run close to sources of interference?

Shielded fire-resistant cable is for circuits that must survive a fire and also transmit accurate signals. These are also the most important circuits in a fire protection system. Which circuits must use fire-resistant cable is defined by the fire protection design and the equipment manufacturer.

Shielded fire-resistant cable is not simply a fire-resistant cable with a screen wrapped around it. The layers must be arranged so that one feature does not weaken the other. From the inside out, the cable has the following layers:

Which screen should you choose for a fire-resistant cable?

The shielding layer affects more than interference protection. It also changes the size, stiffness and termination of the fire-resistant cable.

Whatever type you choose, the screen only works when it is earthed. Earthing at one end avoids circulating currents through earth (ground loops) and suits low-frequency interference (below about 1 MHz). Earthing at both ends with 360° contact blocks high-frequency interference better, but needs a good equipotential earthing system. For fire protection circuits, follow the instructions of the fire alarm panel manufacturer.

Learn why shielded cables still face EMC problems

4.2 Interference protection parameters

There is no pass/fail test for interference protection like IEC 60331. To assess a cable's interference protection, check these parameters in the datasheet:

• Screen type: foil, braid or both; an overall screen or individually screened pairs.
• Braid coverage (%): typically about 60–95%.
• Twisted pairs: whether the cores are twisted in pairs.
• Capacitance (pF/m): a screen increases capacitance, and addressable fire alarm panels have a capacitance limit per loop.

HELU offers two types of cables for fire protection systems , manufactured in accordance with German standard designations. The first group is shielded fire alarm cable for signal circuits. The second group is 0.6/1 kV safety cable for power circuits that must keep working during a fire.

7.1 Shielded fire alarm cables

All products in this group carry the code (St), which means they have a metal foil screen. They differ in sheath material and in their circuit integrity rating.

The J-H and JE-H series feature halogen-free housings, making them suitable for crowded areas. Check out our entire product catalog!

7.2 0.6/1 kV safety cables with functional integrity

This group is for power circuits that must keep working during a fire, such as fire pumps, smoke extraction fans and emergency lighting. They belong in the "fire-resistant only" box of the table above, unless the circuit runs near interference sources.

View the entire catalog at Safety Cable With Functional Integrity – 0.6/1 kV.