In brief
Static charge building up during solvent transfer, powder conveying and filling produces a spark that can ignite flammable vapour or a dust cloud. The basis of protection is connecting every conductive part to each other (equipotential bonding) and connecting this system to earth (earthing). You complete this by removing non-conductive containers and using the correct bag type, antistatic footwear and clamp checks.
Why is static electricity an ignition source?
When two surfaces rub together and separate, when a liquid flows through a pipe or when powder passes through a hose, charge separates. If the charge cannot find a path to earth, it builds up in the container, the machine or the human body. When the accumulated charge jumps to a nearby earthed part, a small but hot spark forms. If there is vapour from a solvent such as thinner, acetone or ethyl acetate, or a cloud of flour, plastic or metal dust in the atmosphere, this spark is enough to start an explosion. Static electricity controls are therefore a topic to be handled separately from the electrical installation: there are no cables, sockets or faulty devices involved; the hazard arises from the process itself.
Art. 6 of the Regulation on the Protection of Workers from the Hazards of Explosive Atmospheres requires ignition sources, including static electricity, to be taken into account when assessing explosion risk. Section 2.3 of Annex 2 requires electrostatic discharges to be taken into account where workers or the working environment may carry or generate static electricity, and requires workers to be provided with personal protective equipment made of suitable materials. We explain zone classification and the document side in our article on ATEX and the explosion protection document; this article focuses on how to make the connections on site.
Which operations carry a risk of static electricity explosion?
For a static electricity explosion, three conditions must occur at the same time: a process that generates charge, an insulated part in which the charge can accumulate and an ignitable atmosphere. The table below summarises operations commonly encountered in paint, chemical, plastics and furniture businesses in Bursa and the surrounding provinces.
| Operation | How is charge generated? | Mistake we often see |
|---|---|---|
| Transferring solvent from a drum to a container | Liquid flow and splash filling | Only the drum earthed, the receiving container left floating |
| Filling a tank from a tanker or IBC | High velocity in the pump, filter and pipe | Starting the pump before the tanker clamp is attached |
| Carrying thinner in a plastic jerrycan | Charge trapped in an insulating container | Using a non-conductive jerrycan for flammable solvent |
| Discharging powder from a bag into a reactor or mixer | Powder sliding on the bag and hopper surface | Discharging powder with a plastic bag into a vessel containing solvent vapour |
| Pneumatic powder conveying | Powder rubbing against the pipe wall | Plastic hose sections, unbridged flanges |
| Belts, conveyors and reel winding | Separation of surfaces | Earthing brush removed and not refitted |
| Worker movement | Friction between shoe soles and the floor | Insulating soles, insulating coating on an epoxy floor |
What the jobs in the table have in common is that the charge cannot flow to earth through a conductive path. All the measures aim to create this path or to slow down the generation of charge.
The difference between earthing and equipotential bonding
The two concepts are often confused on site:
- Equipotential bonding: Connecting two conductive parts, for example a drum and a receiving container, to each other with a cable and clamp. The aim is to ensure there is no potential difference, and therefore no spark, between the two parts.
- Earthing: Connecting the bonded system to earth through the business's earthing conductor. The aim is for the accumulated charge to flow away safely.
If you only bond and do not earth, the two containers stay at the same potential but may be charged relative to a nearby earthed steel structure or a worker. If you earth only one and leave the other floating, the spark forms at the floating container. The correct practice is both together: bond first, then earth. In practice, a very low resistance is expected between metal parts; guidance documents such as IEC TS 60079-32-1 and NFPA 77 generally use a limit of 10 ohms for the bonding resistance of metal equipment. The measurement method and acceptance value should be written in your business's explosion protection document.
Earthing during solvent transfer: step by step
For every flammable liquid transfer from a drum, IBC or tanker, turn this sequence into a written instruction:
- Check the containers. Flammable solvent is only transferred into containers that are metal or manufactured as conductive. An ordinary plastic jerrycan accumulates charge and cannot be earthed.
- Attach the clamps. First connect the source container to the main earth bar, then connect the receiving container to the source container. The clamp teeth must reach the metal under any layer of paint, rust or dirt.
- Verify the connection. If there is an earthing system with an indicator light or interlock, wait for the light to come on. If a simple clamp is used, visually check the cable and end connections.
- Extend the filling pipe to the bottom. Pouring the liquid in free fall from above generates both vapour and charge. The filling pipe should be close to the bottom of the container, and the flow should be kept slow at the start.
- Do not remove the connection during transfer. The clamp is only removed after the flow has stopped and the container has been closed, in the reverse order of attachment.
- Manage the environment. Ventilation is running in the transfer area, no tools that could be an ignition source are present and a spill kit is ready.
For storing flammable liquids and room layout, see our article on storing flammable liquids, and for the health effects of vapour, our article on solvent vapour exposure.
Static electricity controls in powder conveying and filling
There are two separate hazards in powder operations: the cloud of the combustible powder itself and solvent vapour in the vessel into which the powder is discharged. Because the latter ignites with less energy, adding powder to a vessel containing vapour requires particular care.
- Bulk bags (FIBC): A Type A bag has no static protection and is not used in a flammable atmosphere. Type B is only suitable for environments with combustible dust but no flammable vapour. Type C is a bag with conductive threads and must be connected to an earthing point. Type D is designed to dissipate charge without requiring earthing. The type declared by the supplier on the bag must match the environment in your risk assessment.
- Small bags: Manually discharging powder from a plastic bag into a reactor or mixer containing solvent vapour is one of the most dangerous known habits. Closed feeding, inert gas or a sequence in which the powder is added first and then the solvent should be considered.
- Pneumatic conveying: Pipes must be conductive and there must be bridging cables between flanges. For flexible hoses, a conductive wire or conductive material is chosen.
- Filters and silos: The earthing connections of conductive filter bags must be refitted after maintenance.
We cover the combustion and explosion behaviour of dust in detail in our article on dust explosion risk.
Antistatic footwear, clothing and floors
The human body also accumulates charge like a container. The spark that forms when a worker walking in shoes with insulating soles reaches for a solvent container can be enough on its own for ignition. Therefore:
- Antistatic footwear: Under EN ISO 20345, the resistance of footwear marked "A" is between 100 kilohms and 1,000 megohms; classes S1 and above are antistatic. For details, see our article on safety footwear classes.
- Floors: Antistatic footwear only works if the floor can also conduct the charge. An insulating epoxy coating, rubber mats or thick dirt stuck to the shoes cancel the protection.
- Clothing: In hazardous areas, workwear with antistatic properties is preferred over synthetic outer layers that accumulate charge; clothing is not taken off in the hazardous area.
- Gloves: Even if the footwear is earthed, a metal funnel held with insulating gloves may remain insulated. Funnels and scoops should be bonded separately.
ESD footwear is designed to protect electronic components; check the manufacturer's declaration as to whether it can be used in an explosive atmosphere. Choose footwear and clothing by area, not by person: visitors and maintenance teams entering the hazardous area are subject to the same rule.
Inspection, maintenance and records
A system set up against static electricity silently deteriorates if it is not maintained. Plan inspections on three levels:
| Check | Frequency | Who carries it out? |
|---|---|---|
| Clamps, cables and indicator light | Before every transfer | The operator carrying out the transfer |
| Bridging cables, flange connections, earthing points | At intervals set by the business, and always after maintenance | Maintenance team, with a checklist |
| Internal electrical installation and earthing installation | At least once a year (Work Equipment Regulation Annex III) | Authorised person registered on EKİPNET |
The annual periodic inspection covers the business's earthing installation; the day-to-day condition of the static connections you have added to machines, tanks or hoses is monitored with your own checklist. We explain the installation side in our article on earthing measurement and residual current devices; you can track inspection dates with the OHS calendar tool.
When a new solvent, a new filling line or a different type of bag is introduced, the change should be entered into the risk assessment and the explosion protection document. At Bursa Sistemer, we examine transfer and filling points one by one on site at chemicals and plastics businesses and write the instructions together with the operators.
Frequently Asked Questions
Are earthing and equipotential bonding the same thing?
No. Equipotential bonding connects conductive parts to each other so that there is no potential difference between them. Earthing connects this bonded system to earth so that the charge can flow away. In solvent transfer, the two are applied together.
Can I transfer thinner with a plastic jerrycan?
An ordinary plastic jerrycan accumulates charge and cannot be earthed; it should not be used for flammable solvents. Containers that are metal or manufactured as conductive and declared for this purpose should be preferred, and bonding and earthing should be carried out during transfer.
Is antistatic footwear enough on its own?
It is not enough. For the footwear to work, the floor must also be able to conduct the charge. In addition, containers, funnels and equipment must be bonded and earthed separately; footwear only targets the charge on the human body.
Is static earthing included in the annual periodic inspection?
The internal electrical installation and earthing installation are inspected at least once a year under Annex III of the Work Equipment Regulation. The day-to-day condition of clamps and bridging cables at transfer points should be monitored with the business's own checklist.
A risk assessment that identifies and scores hazards on site, links each measure to an owner and a date, and has a clear renewal schedule.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.