Description
The STS 300 is a static ground monitoring system that continuously verifies a low-resistance path from the grounding clamp to a verified ground during material transfer. Two dry relay contacts interlock pumps, valves and alarms, so a transfer cannot start, or continue, without a confirmed ground. Special Technical Services, founded in 1976 and based in Budd Lake, NJ, builds the STS 300 in the USA in single-point and multipoint versions.
What the STS 300 does
A grounding clamp makes the physical connection, but it cannot tell the operator whether that connection is still electrically sound. The STS 300 uses an intrinsically safe monitoring circuit, a grounding cable and a clamp to check the path from the clamp to ground for the whole operation, against a resistance-to-ground setpoint of less than 5 ohms per NFPA 77.
- Continuous monitoring. The STS 300 checks the ground path from hookup to disconnect, not only at the start of the transfer.
- Automatic interlock. Two interlocking dry contacts connect to filling equipment and controls. If the system cannot verify a proper ground, the contacts change state and connected equipment is halted until the ground is re-established. Common integrations start or stop pumps, open or close valves, and sound alarms.
- Visual status. Indicator lamps show the grounding status at a glance: green when a proper ground connection is made, red when the circuit is broken or the ground is interrupted.
Indicator light status
| Red | Green | Status |
|---|---|---|
| On | Off | Ground connection not verified. Controller is non-permissive. Do not begin operation. |
| Off | On | Ground connection verified. Controller is permissive. Operation may begin. |
| Off | Off | No power to the grounding system. Controller is non-permissive. Do not begin operation. |
Configurations
STS 300 single-point
The standard STS 300 monitors one grounding connection, which suits most vehicle and equipment grounding, such as tanker trucks, drums, railcars and barges.
STS 300 Multipoint
The STS 300 Multipoint gives operators a single control device for grounding up to four pieces of equipment at the same time. It is offered in 2-clamp or 4-clamp configurations and only operates when all cables are connected, so every piece of equipment must be grounded before the transfer is permitted. Multipoint clamps come with 10 ft or 25 ft straight cable.
Power, lights, cables and clamps
- Source voltage: 120 VAC, 240 VAC, 12 VDC or 24 VDC.
- Indicating lights: LED or incandescent.
- Cable: straight or self-coiling cord. Coil cable and straight cable are offered in 25, 35 and 50 ft catalog lengths, and 100 ft is available in straight cable only.
- Clamps: aluminum, bronze and dual ball plug clamps, including the K78160A, K78160ASRS with strain release, REB, VUD, CB-250IP bronze C-clamp, the StaticMag magnetic clamp and the Dual Ball Connector.
Options
- Retractable cable reels: standard duty or heavy duty reels protect the ground cable; reels use straight cable, not coil cable. See accessories.
- Deadman control: requires the operator to activate a handle, in addition to an acceptable ground, to enable the permissive interlock circuits. An optional timing feature requires the operator to reactivate the handle after an adjustable delay of 15 seconds to 10 minutes.
- Clamp stowage plate: a single clamp stowage plate or multipoint stowage plate keeps the clamp off the ground when the system is not in use.
Specifications
| Specification | STS 300 and STS 300 Multipoint |
|---|---|
| Hazardous location listing | UL Listed for Class I, Division 1, Group D and Class II, Division 1, Groups E, F and G hazardous locations |
| Certification standards | UL 913 and UL 1203 |
| Monitoring circuit | Intrinsically safe |
| Enclosure | NEMA 7, 8 and 9 explosion-proof; cast aluminum, powder coated; breather/drain included |
| External hardware | Stainless steel |
| Source voltage | 120 VAC (102 to 130 VAC, 50/60 Hz); 240 VAC (204 to 240 VAC, 50/60 Hz); 12 VDC (9 to 15 VDC); 24 VDC (18 to 30 VDC) |
| Power consumption | 5 watts energized |
| Fused protection | 5 amps |
| Resistance to ground setpoint | Less than 5 ohms per NFPA 77, continuously monitored |
| Output contacts | 2 voltage-free (dry) Form C contacts (DPDT) |
| Indicating lights | LED or incandescent; red = non-permissive, green = permissive |
| Overall size | 14 x 11 x 7 in |
| Average weight | 23 lb (10 kg) |
| Mounting | Four 5/16-18 threaded holes on the back of the enclosure, spaced 5 x 4.5 in |
| Integrity ground cable | Service grade, 2 x 14 AWG stranded copper, 8 ft |
| Operator cable | Service grade stranded copper: coil 2 x 16 AWG; straight 2 x 14 AWG; multipoint 2 x 16 AWG |
| Clamp contacts / body | Stainless steel contacts; aluminum or bronze body |
| Multipoint configuration | 2 or 4 clamps |
| Batteries | None |
Full wiring, installation and maintenance details are in the STS 300 installation and operating manual (PDF). Product sheets are available for the STS 300 and the STS 300 Multipoint.
Installation and service life
Each STS 300 is fully assembled and tested before shipment. Installation (mounting, power and any auxiliary control circuits) should be performed by a licensed, qualified person to the codes that apply at the site, with the unit at the operator’s eye level. A seal fitting (not included) is required where the supply conduit enters the enclosure.
Every STS 300 component is replaceable. STS stocks replacement parts including circuit boards, relays, lamp assemblies, lens caps, bulbs and clamps. Systems installed in the 1980s are still supported, and the enclosure size and mounting have stayed the same for 40 years, so older units can be replaced on the same mounting.
Applications
The STS 300 is used where flammable liquids, vapors or combustible dusts are transferred:
- Petroleum refining
- Chemical processing
- Rail car fill stations
- Jet fuel fill stations
- Vacuum trucks
- Marine terminals
- Oil rigs
- Grain and powder processing
- Food and beverage
- Paper manufacturing
STS 300 compared with a grounding clamp alone
A grounding clamp and cable make contact but do not report whether the contact is still good. Paint, rust, residue, vibration or a bumped clamp can raise the resistance of a connection that looked fine at hookup, with no warning to the operator.
| Capability | Clamp and cable alone | STS 300 |
|---|---|---|
| Makes a physical connection | Yes | Yes |
| Verifies the ground path during the transfer | No | Yes, continuously, against a less than 5 ohm setpoint |
| Shows grounding status to the operator | No | Yes, red and green indicator lamps |
| Stops pumps or closes valves if the ground is lost | No | Yes, through two dry interlock contacts |
For more detail, read how to choose a static ground monitoring system for flammable and combustible materials, how to ground a tanker truck, static grounding requirements for flammable liquids and static grounding standards compared.
Ordering an STS 300
Each STS 300 is identified by a catalog number that encodes the system, voltage, indicator light, cable type, cable length, clamp and options. For example, G2LC25KA is a 120 VAC system with LED lights, 25 ft coil cable and a K78160A clamp. Use the ordering guide to build a catalog number, or request a quote and STS will help you specify the right configuration. You can also call +1 609-259-2626.
Frequently asked questions
What happens if the ground connection is lost during a transfer?
The indicator changes from green to red and the two interlocking dry contacts change state, which can stop pumps, close valves or sound alarms. Connected equipment stays halted until a proper ground is re-established.
What is the difference between the STS 300 and the STS 300 Multipoint?
The STS 300 monitors one grounding connection. The STS 300 Multipoint monitors two or four connections from one control unit and only permits operation when all cables are connected.
Is the STS 300 rated for hazardous locations?
Yes. It is housed in a NEMA 7, 8 and 9 explosion-proof enclosure and is UL Listed for Class I, Division 1, Group D and Class II, Division 1, Groups E, F and G hazardous locations.
What power sources can the STS 300 use?
The STS 300 is available for 120 VAC, 240 VAC, 12 VDC or 24 VDC. The voltage is marked on the front nameplate.
Which clamps work with the STS 300?
Aluminum, bronze and dual ball plug clamps are available, including the K78160A, K78160ASRS, REB, VUD, CB-250IP, StaticMag and Dual Ball Connector. Cables and clamps can be replaced in the field.
