Why is isolation required in fieldbus power conditioners?

Maintaining isolation between the Fieldbus pair and plant ground is critical to the reliability of FOUNDATION fieldbus™ systems.

Isolation has been a requirement of FOUNDATION fieldbus™ since its inception. It was written into the ISA S50.02 standard and later adopted into the IEC 61158-2 standard. The primary reason for this was to reduce the chance of an outside electrical interference affecting the FOUNDATION fieldbus™ signals.

As we have gained field experience, it has become apparent that there are other issues that can adversely affect a FOUNDATION fieldbus™ system when isolation is lost.

What cable lengths can be supported using Redundant FISCO?

* - Limited by total segment length

Doesn’t the FISCO standard say that there should only be one source of power in a FISCO network, mak

No. This clause in the IEC60079-27 standard is intended to mean that the maximum values of Uo, Io and Po for the power supply must not be exceeded on any segment, and that the power must be connected at only one point on the bus. MTL’s solution complies with both these requirements because it provides power onto the segment at only one point (at the power supply terminals), and the Uo, Io and Po values for the redundant output will be certified to comply with the maximum values specified for FISCO.

Can the existing 932x-SC Spur Connectors be used to connect to ‘Entity’ certfied fieldbus devices, a

Yes, these can be used on Redundant FISCO segment in exactly the same way as for simplex.

Can the modules be ‘hot-swapped’ to allow replacement of failed modules without losing power or comm

Yes. The power supply and Supply Arbitration Modules may be easily ‘hot swapped’ without powering down the system, and without causing loss of power or communications.

Does the Redundant FISCO power supply work with existing Intrinsically Safe Megablock wiring hubs, o

Redundant FISCO works with MTL’s existing range of IS Megablocks (types F245 – F271).

Why are there two Supply Arbitration Modules per fieldbus segment on a redundant FISCO system?

To eliminates single points of failure for complete redundancy. If one ‘SAM’ fails, the system continues to provide power to the fieldbus segment.

Will the output specification of Redundant FISCO be the same as the existing 912x-IS power supplies?

There will be a slight reduction in available output current and voltage compared with MTL’s existing range of ‘simplex’ power supplies, but this will result only in a small change to the maximum segment length or device loading.

What’s inside the ‘Supply Arbitration Modules’ of a redundant FISCO system? Are there relays inside?

Each Supply Arbitration Module (SAM) contains electronic circuits to monitor the output of one FISCO power supply, and to switch the output of that power supply to the field segment. The switching is solid-state, so there are no relays. The two modules communicate with each other by ‘handshaking’ to make sure that they can never both be ‘on’ at the same time.

Is the change-over to redundant FISCO ‘bumpless’ ?

Yes. Interruption of fieldbus communications is momentary, and is accommodated by the re-transmission mechanism that is implicit in the Foundation fieldbus protocol. There will therefore be no loss of control, and the plant operator will observe no loss of data.

What allowance do I need to make if using a SpurGuarded™ Megablock with a FISCO power supply?

The F240-F259 range of Relcom SpurGuard™ed Megablocks are designed for IS applications and are ideal for providing wiring connections on a FISCO segment. The F251 8-way Megablock normally draws 3.5mA plus the current for any devices connected. In the event ot a device having a short circuit, that device will now draw a maximum of 42mA instead of its normal current (e.g. 15mA).

For example, consider a segment with sixteen 15mA devices connected via two F251 8 way Megablocks powered by a 9122-IS IIB FISCO power supply. The segment will normally require (16 x 15 + 2 x 3.5 =) 247mA which is less than the 265mA 9122-IS design current.

If a single spur has a short cicuit, the current consumption will be 247 + 42 –15mA = 274mA, which is still less than the 285mA current limit of the 9122-IS, so this segment design is OK.

Additionally, you will need to check that the voltage at the devices will always be greater than 9V. Some designers use a minimum of 10V at a device to provide some margin.

An MTL-Relcom segment calculator is available which will automatically complete all these calculations for you. With typical type A cable (0.8 sq.mm, 44 ohm/loop km) and all devices at the end of the trunk connected by 30m spurs the maximum permitted trunk length would be 330m. Under the same conditions but with 1.5 sq.mm, 22 ohm/loop km cable, the maximum permitted trunk length would be 660m.

On the few occasions when longer trunks are required, either fewer field devices could be connected to the segment or, alternatively, the FISCO power supply could be mounted in a Zone 2 hazardous area, close to the field devices.

What allowance do I need to make for the fieldbus signal when designing a segment?

No allowance needs to be made for the fieldbus signal if the segment uses a FISCO or FNICO power supply. As the fieldbus signal is symmetrical, the terminators act as a current source/sink for the fieldbus signal, so no extra current is drawn from the power supply.

What allowance do I make for field device inrush current when designing a FISCO/FNICO segment?

When designing a segment with a FISCO or FNICO power supply no allowance needs to be made for inrush current. The IEC61158-2 fieldbus standard allows fieldbus devices to draw up to 20mA above their design current for up to 20ms on start up.

The current limit of the FISCO and FNICO power supplies is at least 20mA above the design current used for segment calculations. This means that if a device is disconnected from a fully loaded segment; on reconnection, it can briefly draw up to 20mA above the design current, without affecting the normal operation of the segment.

The FISCO and FNICO power supplies have a soft start feature to ensure the fieldbus segment starts up. For example for a segment with 16 x 15mA devices powered by a 9122-IS IIB FISCO power supply, the segment will normally require 240mA which is less than 265mA 9122-IS design current. On start up the segment could have an initial current draw requirement of 240mA + 16 x 20mA in rush current or 560mA. The 9122-IS will supply at least 285mA for up to 1 sec, which is long enough to power up all the field devices. In the MTL fieldbus test laboratory we have proven that the segment will start up when fully loaded with devices with the highest in rush currents, confirming that with MTL power supplies no additional allowance should be made for in rush current in segment calculatlions.

Can I use a field device that is not FISCO certified with the MTL FISCO power supplies?

Yes. Field devices that are only certified to the entity profile can be used with the MTL FISCO power supplies by including a spur connector in the connection to that device. The entity spur connector is usually located in the fieldbus junction box.

What FISCO certified field devices are available today?

All major automation vendors have committed to FISCO certifying their intrinsically safe field devices. The following manufacturers have a wide range of FISCO certified field devices

ABB
Emerson Process Management
Endress + Hauser
Flowserve
Krohne
Metso
Vega
Yokogawa

Infraserv of Frankfurt, Germany carried out fieldbus interoperability testing with 43 field devices from a wide range of manufacturers and reported that 83% were FISCO certified by the end of 2002. More have been certified since then so there is a wide choice of FISCO certified devices available today.