The 10-Gram Part Nobody Puts on the BOQ: DEHN BUStector BT 24

Walk through a KNX distribution board after a storm season and you will usually find the mains surge protection, if the electrician fitted it. What you rarely find is anything protecting the bus.

There is a small part made for exactly that job: the DEHN BUStector BT 24 (part number 925 001). It weighs 10 g. It is easy to leave off a quote, and easy to forget about until something has already gone wrong. This post covers what the datasheet says it is, where DEHN itself says it goes, what it will not do, and how I decide whether a project needs one.

What it is

DEHN describes the BT 24 as a surge arrester with a KNX bus terminal design, adapted to the immunity of KNX/EIB systems. It carries EIBA certification (No. Z 32/1399/95) and is tested to IEC 61643-21. The datasheet gives these figures:

• SPD class B, nominal voltage 24 V, maximum continuous d.c. voltage 45 V

• Nominal load current 6 A

• Lightning impulse current (10/350 µs): 1 kA per line

• Nominal discharge current (8/20 µs): 5 kA per line

• Voltage protection level at nominal discharge current: 1200 V line to line, 650 V line to protective earth

• Cut-off frequency 70 MHz, capacitance 10 pF or less line to line and line to earth, so it does not load the bus signal

• Spring contacts and connecting leads, earthed through a 0.75 mm² lead 200 mm long

• IP 20, operating range -40 to +80 degrees C, 10 g

Two of those numbers matter most. The first is the 5 kA discharge rating per line, which I read as sized for induced surges rather than a direct strike. The second is the earth lead. DEHN specifies 200 mm, and I would keep it to that length.

Where DEHN says to fit it

DEHN's product page gives two examples. On the KNX power supply unit, it sits in the standard bus terminal slot. On a bus coupling unit, it sits on a bus terminal in the mounting panel of a cable duct.

DEHN's white paper on KNX surge protection (WP010, 2015) lists the BT 24 in its distribution board arrangements and says surge protective devices "always have to be installed at the distribution board of the KNX system". A second DEHN white paper adds that lightning equipotential bonding is needed where a bus cable enters a building, and that all cables crossing from lightning protection zone 0A to zone 1 must be protected by lightning current arresters.

Why you might install it

The white paper's warning is direct. KNX systems may be destroyed by coupling despite their high dielectric strength, and a line segment can run up to 1000 m. Long runs are the obvious risk. On a large property, a gate house, a pool plant room or an outbuilding can put several hundred metres of bus cable outside the main house. A nearby strike induces a surge in a long conductor, and a bus cable is one.

DEHN also notes that loops make things worse. If the bus cable and the low-voltage cable feeding the same devices are not routed close together, the loop acts as an antenna. A metal pipe connected to the main earthing busbar can create the same loop.

So the cases where I would spec it are:

1. A bus line that leaves the building to reach another building or an outdoor device.

2. A site with long bus runs, especially near the 1000 m segment limit.

3. A client in a high-lightning area who has already paid for actuators, a bus power supply and a gateway that all hang off the same bus.

I covered the lightning side in my earlier post on surge protection in storm season. In that post, I said that where a bus line leaves a building, my own practice is one arrester at each end. That is my reading of DEHN's zone-boundary advice, not a quote from it.

Why you might not

DEHN's own paper says short bus cables with isolated sensors are unlikely to be destroyed, and that it is not necessary to install surge arresters directly at the bus devices. A small flat with a short line inside one building is a different case from a rural estate with outbuildings.

If the bus cable runs in a shielded duct or metal pipe earthed at both ends, DEHN says lightning equipotential bonding does not have to be established for the KNX cable beyond the buildings, and surge arresters alone are sufficient. So the design choice there is about the duct and the earthing, not only the part.

What it will not do

It is not a substitute for mains protection. DEHN's guide shows SPDs at the main distribution board and at the KNX distribution board, with mains arresters listed alongside the BT 24. The BT 24 is rated for 1 kA lightning impulse current per line, so a building with an external lightning protection system needs a proper design from someone qualified to do one.

It also cannot fix a layout problem. A wide loop will still pick up energy, and the arrester will only clamp what reaches it.

Why it often does not get installed

This is my view, not from the datasheet.

It does not appear on most quotes because nobody asks for it. It is tiny, it is invisible once the board is closed, and nothing about the system looks different with or without it. Until a storm takes out an actuator, there is nothing to see. That makes it a design-stage decision: if it is not in the specification, it will not be in the board.

I have not put a price in this post. Check the current price from DEHN Africa or your distributor before you quote it, and compare it against the cost of an actuator, not against the cost of nothing.

What I would add to a brief

If you are an architect or designer, a short line is enough: "Surge protection on mains and on any KNX bus line leaving the building." Your integrator and electrician will know what to price.

If you want a second opinion on a planned site, particularly one with outbuildings, just a conversation about what the project needs: wayne@knxlogic.co.za | 082 564 3982 | www.knxlogic.co.za

Sources: DEHN product page, BUStector BT 24 (dehn-africa.com and dehn-international.com, part no. 925 001) and DEHN datasheet 925001 (technical data); DEHN + SÖHNE, white paper "Surge protection for KNX systems", WP010/E/0615 (2015) and white paper WPX019; DEHN lightning protection guide, KNX section (figures 9.10.6 and 9.10.7).