Where the charge comes from
Powder and granules pick up electrical charge as they slide down a chute, through a spout and against woven fabric. Polypropylene is an insulator, so that charge can build on the bag and on the product rather than bleeding away. If it releases as a spark or a brush discharge where a flammable dust cloud, vapor or gas is present, it can ignite it.
Speed, dryness and fineness all push the charge higher. A fast pneumatic fill, low humidity in winter and a powder ground to a fine mesh tend to generate more of it than slow gravity flow of coarse, damp granules. Operators standing on insulating floors, or wearing insulating footwear, can carry charge too, which is why grounding people and equipment is part of the same conversation.
The four letters are a shorthand for how each construction handles that risk. They sit on the label beside the SWL and safety factor, and they are independent of both: a 6:1 bag can be Type A, and a 5:1 bag can be Type C.
The four constructions in one table
Read each row as a description of the fabric and its intended setting, not as a promise. Every one of them still has to be checked against the product, the room and the procedure on the day it is filled.
| Type | What it is and where it is meant to go |
|---|---|
| A | Plain polypropylene with no static protection. For non-flammable products where no flammable dust or vapor is present. |
| B | Fabric with a low breakdown voltage that prevents high-energy propagating brush discharges. Not groundable. For dry flammable powders only where no flammable vapors or gases are present. |
| C | Conductive threads woven through the fabric, connected to ground during filling and discharge. Useless and unsafe if the ground is not connected. |
| D | Static-dissipative fabric that dissipates charge safely without a ground connection. |
Type C depends on the clip
A Type C bag has grounding tabs that the operator clamps to an earthed point before the first pound goes in, and leaves clamped until the bag is empty. The protection exists only while that connection is made. Forget the clip, or clamp it to something that isn't truly earthed, and you have an ungrounded conductor holding charge, which is worse than a plain bag.
That makes Type C a procedure as much as a product. It suits plants with fixed grounding points at every filler and discharger and a routine that operators follow every time. If bags travel to a customer whose site lacks that discipline, the risk travels with them.
Many plants back the habit with hardware: a monitored grounding clamp or an interlock that won't let the filler start until the clamp has closed on the tab and confirmed continuity. Ask your safety team whether your lines need one.
Type D removes the clip from the routine, which is why some plants prefer it where several crews or several sites touch the same bags. Type B removes nothing from the hazard list: it guards against one particular kind of discharge and is ruled out the moment flammable vapor or gas enters the picture. Neither lets anyone skip the assessment.
A walk-through by product and surroundings
Here is the order of questions we work through with a caller. It narrows the field; it doesn't replace the assessment your safety team owns.
A few familiar examples help frame the first question. Quarry sand, gravel, crushed stone and many inert minerals don't burn. Flour, sugar, starch, grain dust and a lot of fine plastic powders are combustible dusts, so a cloud of them near a spark is a fire or explosion risk. Rooms where solvents are mixed, dried or stored can hold flammable vapor even when the bagged product is dry.
- Is the product itself non-flammable, like sand, gravel or many mineral fillers, and is the area free of flammable dust and vapor? Type A, coated or uncoated, is the plain option.
- Is the product a dry flammable powder, with no solvent vapor or flammable gas anywhere near the filler or discharger? Type B, C or D can be considered; Type B is limited to exactly this case.
- Are flammable vapors or gases present, for example from solvents in the same room or a product wet with solvent? Type B is ruled out; look at Type C or D.
- Can you guarantee a working ground connection at every fill and every discharge, including at your customer's site? If yes, Type C is an option. If not, Type C is off the table.
- Does the bag need a liner? Then the liner and bag must be specified together, as the next section explains.
- Has your site classified the area? Follow your site's hazardous-area assessment and the bag maker's documents before choosing.
Liners and static
Many bulk bags carry a polyethylene or polypropylene film liner, or a laminated coating, to keep out moisture or contain fine dust. A film is an insulator in its own right. Dropped inside a Type C or Type D body, an ordinary liner can change how charge behaves at the surface the product actually touches.
So never order the bag and the liner separately when static matters. Ask for the combination, and ask the maker to confirm that this liner in this body has been assessed for the type on the label. If the paperwork covers only the outer bag, that's not enough.
Some makers offer liners built for static-sensitive service, and some bonded or glued-in liners are designed as part of the bag. Whatever you choose, keep the pairing fixed once it is approved. Swapping to a cheaper film later, without the maker re-checking it, quietly undoes the decision your safety team made.
Standards to look up
IEC 61340-4-4 covers electrostatic classification and test methods for FIBCs, and it is where the Type A to D definitions and their test requirements live. NFPA 77 is the recommended practice on static electricity that many US sites use when they work through ignition hazards. ISO 21898:2024 covers general construction, design and testing for bags carrying non-dangerous solids.
We don't reproduce the thresholds from any of them here. Read the current editions, read the bag maker's technical documents, and let the person who signs your hazardous-area assessment make the match. We'll quote the bag that assessment names.
Standards are revised, and a bag specified years ago against an older edition may not match the current wording. When a plant renews its process hazard review, it's worth asking the bag maker whether anything in their test reports has changed since the last purchase.
What a static-sensitive request should include
Tell us the product, whether solvents or flammable gas are present at either end, whether grounding points exist at the filler and at discharge, whether a liner is needed, and the type your assessment calls for. Add the SWL, the top, the bottom and the ZIP of the fill line, and we'll plan the load from there.
If your customer discharges the bags at a different plant, send their conditions as well. The type has to suit both ends of the trip, not only the building where the bag is filled, and a receiver with solvent tanks next to the unloading station changes the answer.
Questions
Can a Type C bag be used without connecting the ground?
No. An ungrounded Type C bag is useless and unsafe. If grounding can't be assured every time, choose a different type with your safety team.
Is Type D always the safest choice?
Not automatically. It removes the need for a ground clip, but whether it suits your product, liner and surroundings is a call for your hazardous-area assessment and the maker's documents.
Will you quote used Type B, C or D bags for static protection?
No. Used bags are only for non-food, non-static-sensitive loads. Wear, residue and handling can change how a bag behaves, so static jobs get new bags.
Does the static type change the safety factor?
No. The type describes static behavior; the safety factor describes lifting strength. Read both fields on the label.
Is Type A fine for sand, limestone or road base?
For a non-flammable product in an area free of flammable dust and vapor, Type A is the plain construction meant for that kind of load.
Sources
Standards named here (ISO 21898, IEC 61340-4-4, NFPA 77) are summarised, not quoted: read the standard and the bag maker's documents before you rely on a rating.