Ethernet-APL Network How Do I Get Started? — Tips for Planning Ethernet-APL Networks

A guest post by André Fritsch

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The work for Ethernet-APL has been mostly completed — now the first market-ready devices are heading towards use in practice. But how do you start building an Ethernet-APL network? The positive: Some things seems new at first glance, but many things become easier.

Example topology for the use of Ethernet-APL in process plants.
Example topology for the use of Ethernet-APL in process plants.
(Source: R.Stahl)

Last year’s Achema was the starting point for the possible use of Ethernet-APL in process plants. The technology is there — now it's going into practice. The starting point is that Ethernet-APL enables Ethernet access right up to field devices in hazardous areas and as an internationally standardised 2-wire solution. In contrast to traditional Ethernet technology, the range of which ends at 100 meters with the copper cable, Ethernet-APL is based on the IEEE-802.3cg standard and can bridge distances of up to 1000 m at 10 Mbit/s. It simultaneously provides a supply for the field devices — an indispensable advantage in the extensive systems used in the process industry. Additionally, field devices can be operated with the intrinsic safety type of protection, which is preferred in this industry.

A consistent Ethernet infrastructure offers many benefits for planning, commissioning and troubleshooting. Changes and modifications, but also new concepts such as the Namur Open Architecture (NOA) or the Open Process Automation Standard (O-PASTM), are significantly quicker and more flexible to implement than today's generally non-homogeneous installations.

Field Switches Tasked with Many Duties

Planning and installation of Ethernet APL segment is no more complicated than a traditional fieldbus installation, for example. So that planners, integrators and installation technicians are supported from the outset, a comprehensive engineering guide was created at the same time as the technical specifications, which covers planning- and cabling-related aspects, as well as explosion protection.

To build an Ethernet APL network and include the field devices, what are known as field switches are needed first. A field switch is quite simply a switch, as is required for any Ethernet installation. In addition to distributing and coupling data streams, it also takes on additional tasks for Ethernet-APL. It supplies the connected field devices with intrinsically safe auxiliary power, which occurs via a spur line, or “spur”. These spurs are limited to a length of 200 m, which limits the installation location of the field devices to the field switch. Depending on the supplier, these field switches can be installed in the control room, Zone 2 or Zone 1. The last conformance tests for field switches are currently being carried out.

Star Topology and Trunk/spur Technology Challenges

Essentially, Ethernet-APL offers two options for networks. On the one hand, Ethernet-APL can be installed in the star topology usually used for Ethernet. This means the field switch is directly connected to a 4-wire Ethernet such as 100BASE-TX over a maximum 100 m distance. If fibre optics are used, what is optionally supported by most field switches, longer distances (2 to 20 kilometres) can also be bridged. This installation has two main advantages: Firstly, planning is very straightforward and only the cable length has to be taken into account. Secondly, up to 250 field devices can be connected per network, as the field switches are supplied with energy separately.

Alternatively, the trunk/spur topology can be used, which are well-known from the fieldbus world. The network transition from 100BASE-TX or 100BASE–FX network occurs via a power switch here, which likewise must be supplied with auxiliary power. The power switch converts a 4-wire network to a 2-wire network. Furthermore, it supplies the Ethernet-APL network via the trunk line. The field switches are connected to the power switch. The field switches, in turn, supply the field devices with intrinsically safe energy. The field switches can be installed up to Zone 1; the power switch is generally located in the control room or in Zone 2. The trunk line can be up to 1000 m in length and is therefore well suited to the extensive systems used in the process industry. Due to the voltage drops on the trunk, around 50 field devices per segment can be connected. Generally, trunk/spur technology is always a compromise between distance and number of devices. Trunk/spur technology therefore requires good network engineering, as well as a sophisticated shielding/earthing concept, above all for extended networks with copper cables.

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A Look at the Cable

The cabling in an Ethernet-APL network requires particular attention. The type A cables (IEC 61158-2) used in fieldbus installations until now can continue to be used; replacing them in existing systems is often linked with high expenditure. Moreover, when using a fieldbus type A cable, the user is always on the safe side, since Ethernet-APL is adjusted to this as regards the requirements of shielding, conductor length and cross section.

You can rely on tried-and-tested method for the connection types, too. Screw or spring clamp terminals, M12 or M8 plugs are still used. Generally, the connection technology is defined by the device.

Port classification has been introduced to simplify installation and limit the number of variants. There are three power classes: Type A has a power of 0.54 W and is for Zone 1 devices; type B is still in planning and will be suitable for high-power Zone 1 devices (power of 1.17 W); and type C with a power of 1.11 W for Zone 2 devices.

Examples from Practice

The following scenarios show how the changing over or setting up an Ethernet-APL network may look. Here, the calculations assume a maximum current consumption of an APL field device of 50 mA. The real values for the future field devices are probably significantly lower (around 30 to 35 mA). It follows that in practice there will be somewhat greater distances and the availability will be verified again.

Migrating a PA System

  • Scenario 1: The scenario is based on a fieldbus installation, in which an individual field device coupler (FF H1 or PA) is used. These were limited to a maximum of 12 devices respectively. A field switch can currently operate up to 24 devices. The spur lines from the field device to the fieldbus coupler are then up to 90 metres in length. This results in a trunk length of up to 800 m during replacement with an Ethernet-APL field switch. It must be noted that field switches have non-linear behaviour regarding the voltage drop. It is recommended to use special tools for the calculation here.
  • Scenario 2: In a plant section, there are maximum twelve fieldbus devices, which are connected to three available field device couplers. The configuration is to be transferred to Ethernet-APL. To do so, three Ethernet-APL field switches are connected to the Ethernet-APL power switch. The length of the spurs are accepted at 80 m each. The distance between the Ethernet-APL field switches is 100 m. Four Ethernet-APL field devices are connected to each field switch. For the configuration, the distance between the Ethernet-APL power switch and the first Ethernet-APL field switch can be 600 metres when using a trunk cable with a cross section of 1.5 mm² (16 AWG) or 1000 metres when using a trunk cable

with a cross section of 2.5 mm² (14 AWG).

New Ethernet-APL Network

  • Scenario 3: The user would like to connect as many Ethernet-APL devices as possible in one segment. Here, Ethernet-APL field switches, Ethernet-APL field devices and the spur lengths must be considered, and from this, the maximum permissible trunk length can be derived. If 16 spur lines for the Ethernet-APL field switches are used, a total of 48 Ethernet-APL field devices can be installed. The spur length is then 200 m and trunk lengths 300 m respectively, for example.
  • Scenario 4: The user would like to determine the number of field devices at a maximum distance with one segment. The scenario assumes a length of 1000 metres between the individual field switches, the length between the power switch and field switch is likewise 1000 metres. With an APL trunk cable with a cross section of 1.5 mm² (16 AWG), up to two Ethernet-APL field devices can be connected to each of the two Ethernet-APL switches. With a cross section of 2.5 mm² (14 AWG), the number of devices increases to three per Ethernet-APL field switch.

Proving Intrinsic Safety is Getting Easier

While the network technology is new territory for some users, the Ex i verification for Ethernet-APL is significantly easier than for current conventional installations. Ethernet is essentially point-to-point or rather port-to-port connections. This is also the case for Ethernet-APL. One energy source is connected to exactly one energy sink via a defined cable. With these framework conditions, the Ex i verification can be carried out exemplarily, i.e. once, on the basis of IEC 60079-25 “Intrinsically safe systems” for all interconnectable devices.

So that not every user or planner has to carry out verification themselves, the Ethernet-APL working group together with the Dekra exam has already provided this proof and documented it in an IEC TS 60079-47: “Equipment protection by 2-wire intrinsically safe Ethernet concept (2-WISE = 2-Wire Intrinsically Safe Ethernet)”. For users and planners, this means that if all devices used are certified according to 2-WISE, which is marked both in the EC Type Examination Certificate and on the device itself, the interconnection is intrinsically safe.

However, it is still necessary to check whether the equipment is also suitable for the initially specified explosive atmosphere, the ports have the correct type of protection (ia, ib or ic) and the installation location is correct. But no calculation or cable dimensions are required.

There is one thing the user can’t escape — the Ex i verification must be appropriately documented in the explosion protection document. If this last piece falls into place, it paves the way for safe operation of the system with Ethernet-APL.

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