• What FISCO certified field devices are available today?

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

• What allowance do I need to make for field device inrush current when designing a segment?

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

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

What FISCO certified field devices are available today?

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

What allowance do I need to make for field device inrush current when designing a 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.

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

What allowance do I need to make if using a SpurGuard™ed 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.