Samson Type 3770 Ex d/Ex i Field Barrier for Positioners

Samson Type 3770 Ex d/Ex i field barrier for valve positioners

The Samson Type 3770 is a field barrier in a flameproof enclosure, marked II 2G Ex d [ia] IIC T6 Gb. It is the interface between non-intrinsically safe wiring from the control room and the intrinsically safe circuits of devices on a control valve in a hazardous area. The datasheet names positioners, smart positioners with HART communication, i/p converters, solenoid valves and limit switches as devices that it can supply. Seven Star LLC supplies the Type 3770 in Oman and the GCC as an independent supplier in Muscat, usually together with smart positioners on oil and gas control valves.

The idea behind the barrier is practical. An intrinsically safe circuit lets a technician connect and disconnect field devices while the plant is live, but the cable back to the controller must also be intrinsically safe. By mounting a flameproof barrier directly at the valve, the long cable can carry a normal 4 to 20 mA signal and the intrinsically safe segment is only the short run from the barrier to the positioner.

Decoding the Type 3770 article code

The datasheet gives an article code of the form 3770-1xxx0xxx. Channel arrangement, thread, enclosure and a special version are selected. The two examples drawn in the datasheet are 3770-1310 with channel 1 floating and 3770-1410 with channel 1 non-floating.

The floating or non-floating choice is made to match the earthing of the analog output of the controller or control system, so check how your DCS or PLC output is referenced before ordering.

Type 3770 technical specifications

How the field barrier works

Channel 1 is built for analog 4 to 20 mA signals and can carry the HART protocol, so it feeds the positioner both its set point and its digital communication. Channels 2 and 3 control limit switches according to IEC 60947-5-6 or Ex i solenoid valves; the datasheet gives the Type 3767 positioner with a 6 V solenoid valve coil as an example. When the barrier is connected to a multi-channel switching amplifier, the channels of the amplifier must not share a common potential, otherwise the limit contacts could interact.

All circuits are connected electrically to internal and external equipotential bonding terminals. For safety, the intrinsically safe circuits must be connected to the equipotential bonding system with the shortest possible link. Channels 2 and 3 are set up as barriers for positive potential, while channel 1 is either floating or for positive potential.

Where the Type 3770 fits in the positioner system

The barrier mounts directly on an intrinsically safe field device through an M20x1.5 male adapter, for instance a Type 3730-3 positioner, so no extra junction box is needed. If wiring is used instead, the cable ends must go to an Ex i junction box. Devices with HART need an adaptation which is available with Type 3730-3 and Type 3730-6 positioners. Related Samson parts on this site include the Type 3730-3 and 3730-1 positioners, the TROVIS 3793 and 3731-3 positioners and the Type 3767 positioner.

Selection checklist for a Type 3770

• Which devices will be connected: positioner, i/p converter, solenoid valve and/or limit switch, and how many of each.
• Whether HART communication is required through channel 1.
• Controller output earthing: floating or non-floating channel 1.
• Cable entry: ½ NPT aluminium or M20x1.5 stainless steel, and the cable gland that is approved for the enclosure.
• Enclosure material: painted die-cast aluminium or AISI 316 stainless steel for offshore or coastal sites.
• Required certificates (ATEX, EAC) and local acceptance rules.
• Check that the entity parameters of the field devices are compatible with the barrier values listed above.

Installation, commissioning and maintenance of the 3770

The barrier must be connected to the equipotential bonding system before power is applied. Install the non-intrinsically safe cable through a conduit system or a certified metal cable entry. At commissioning, verify loop resistance against the series resistance values, verify the 4 to 20 mA signal and, with HART, communicate with the positioner through the barrier. Keep the enclosure cover threads clean and lightly greased with an approved grease, and never open the Ex d terminal compartment while the circuit is live.

Typical applications of the Type 3770 in Oman and the GCC

Oil and gas wellhead and gathering stations, gas processing plants, refineries and petrochemical units commonly classify areas near control valves as hazardous, and many plants prefer Ex d at the junction with intrinsically safe field devices. Stainless steel enclosures withstand coastal humidity and salt, and the –45 to +60 °C range covers desert nights and hot afternoons at most sites.

Information to send with your Type 3770 RFQ

• Device list to be connected (positioner type, solenoid valve voltage, limit switches).
• Hazardous-area classification, zone, gas group and temperature class.
• Enclosure material and cable entry thread.
• Controller output type: floating or non-floating.
• Certificates required for your project.
• Quantity and delivery site.

Documents and quotation

The Samson data sheet T 8379 for the Type 3770 is attached on this page. Send the device list and the hazardous-area details and we will confirm availability and lead time. Seven Star LLC is an independent supplier and does not claim any official status with the manufacturer.