LON Network - Cable Transient Suppression
EcoStruxure Building OperationTAC IA Series, TAC Vista
LON networks that extend beyond the walls of a single building
Surge arrestors dissipate transient energy before the cable can provide a low resistance path to the building ground, preventing potential electrical damage.
Click here to download the white paper on this topic: Cable Transient Suppression
It is recommended that a Transient Suppression protection device be installed on the LON communications cable at any point where the cable enters or exits the building. The LON is frequently used to connect a collection of controllers in one building with a collection of controllers in one or more other buildings in a campus. The LON may also extend outside the protection of an office building when servicing communications to roof-top equipment and/or outside monitoring equipment.
At any location where the LON cable penetrates and extends outside the building compartment, it is recommended that a gas discharge type lightning arrestor be installed across the LON conductor pair and earth ground as shown below. This should be positioned as close as possible to the wall penetration location. The objective is to keep the cable distance as short as possible before the cable suppression is provided.
It is important that a heavy gauge earth ground connection be provided to the gas discharge protector with as short a length as possible. Here we want to dissipate the transient energy as soon as possible before the cable extends on into the facility and provide a low resistance path to the building ground.
SA2-U8 Surge Arrestor
TAC provides a Model SA2-U8 Surge Arrestor which can be used for this purpose. This SA2-U8 device is a gas discharge arrestor packaged in a small DIN rail mountable enclosure. The SA2-U8 is UL864 listed and is the only device that can be used on TAC UL864 listed system. It must be installed on the LON cable at any entry/exit locations.
Other Gas Discharge Suppressors
On non-UL864 systems, the SA2-U8 may also be used, or the installer may use any one of a number of available 90V gas discharge tube devices. These are typically tubular shaped components and since you need two tubes (one for each of the two LON conductors) a dual element three lead discharge tube is recommended. The device’s leads may be attached to a standard screw terminal block and mounted in most any size electrical box. A collection of some of the suitable gas discharge devices that could be used is as follows:
There are other acceptable part numbers available from each of these manufacturers with variations provided in the lead configuration and lengths while maintaining the same 90V protection rating.
Devices To Avoid
It is not recommended that you use any semiconductor type (Transzorb / TVS / Diode) protection devices. Their higher speed and lower clamping voltage characteristics is not necessary with the inherent protection offered by the FTT-10 and FT-X1 LON transceivers. Such semiconductor based transient suppression devices frequently add some undesirable capacitance to the network which impedes communication performance and most inject signal clamping at voltage levels much lower than necessary or desirable for the Free Topology FTT-10 and FT-X1 based LON networks.
The common mode noise immunity of the FTT LON should be much better than that provided by the typical RS485 network. This should provide more tolerance for electrically noisy environments. However, it has been observed that some semiconductor based network protection units have been used that can drastically degrade communications performance. One such device is the Model 91287 from Atlantic Scientific Corp. which was identified by the manufacturer as a LON System protector. This device clamps the signal at one diode drop below ground level which virtually destroys the LON’s common mode noise tolerance. The presence of one of these devices has been found to be the cause of communications problems in a system operating is an electrically noisy environment. This type of communications impairment may be encountered immediately. In other instances, LON retries may cover the problem and the system may operate for extended period of time before environmental or equipment changes raise noise levels above the tolerable threshold. This threshold is now much worse than RS485 networks. This device, and any similar, should be avoided on any LON systems.