Replace a bus plug stab insulator when it is cracked, chipped, burned, marked with carbon tracks, discolored by heat or deformed, or when it no longer holds the stab firmly. The stab insulator is the plastic housing around the copper stabs on the back of a bus plug. It keeps the phases apart at the point where the plug connects to the busway. BRAH Electric lists seven stab insulators, for General Electric Type FVK and Spectra plugs and for Siemens / ITE BD Series plugs, shown in the table below.
Signs that a stab insulator needs replacing
- Cracks, chips or missing sections. Any break in the housing shortens the insulating path between phases or to the enclosure.
- Carbon tracking. Thin dark lines across the surface are paths burned by leakage current. They conduct, and they grow.
- Heat discoloration, blistering or melting. Browned or distorted plastic means the stab inside has been running hot.
- A loose stab. If the stab rocks or slides in the housing, the insulator is no longer holding it in line with the bus bar.
- Contamination that will not clean off. Oil, metal dust or moisture staining that has soaked into the surface.
An insulator showing any of these is replaced, not repaired. Tape, sealant and adhesive do not restore the insulation the part was molded to give.
What a stab insulator does on a bus plug
A bus plug connects to a plug-in busway through stabs, the copper fingers that clamp onto the bus bars. The stabs for the three phases, and the neutral on a 4-wire plug, sit close together where they pass through the back of the plug. The stab insulator surrounds each stab, holds it in position and separates it from its neighbors. The catalogue describes the part plainly: a plastic housing that provides insulation protection between phases.
Because the insulator also locates the stab, a damaged one affects the connection as well as the insulation. A stab that sits out of line meets the bus bar at an angle, grips with less pressure and heats up. That heat then attacks the plastic further.
BRAH stab insulators and the plug family each suits
These are the parts in the stab insulators sub-category. Each is sold individually as the plastic housing only; the copper stabs are sold separately. All are listed for both fusible and breaker type plugs of the family shown, and for the BRAH substitute plug type named in the fourth column.
| BRAH part | Busway brand | Plug family | OEM plug type | BRAH plug type | Stab position or mounting detail | Plug amperage (catalogue) |
|---|---|---|---|---|---|---|
| FVKSTABINS | General Electric | Type FVK (Flex-A-Plug) | FVK | BEK | A, B and C phase stabs | 30A – 800A |
| FVKSTABINS4 | General Electric | Type FVK (Flex-A-Plug) | FVK | BEK | 4th-wire neutral stab only | 30A – 800A |
| SBCAP | General Electric | Spectra Series | SB | BEB | Insulator insert for A, B, C and neutral stabs | 30A – 800A |
| BOSSTABINS1 | Siemens, ITE, Gould, Bulldog | BD Series | BOS | BES | Mounting hole offset from the notch cut-outs | 30A – 200A |
| BOSSTABINS1H | Siemens, ITE, Gould, Bulldog | BD Series | BOS | BES | Mounting hole in line with the notch cut-outs | 30A – 200A |
| BOSSTABINS2 | Siemens, ITE, Gould, Bulldog | BD Series | BOS | BES | Mounting hole offset from the notch cut-outs | 400A – 600A |
| BOSSTABINS2H | Siemens, ITE, Gould, Bulldog | BD Series | BOS | BES | Mounting hole in line with the notch cut-outs | 400A – 600A |
General Electric Type FVK: phase and neutral are different parts
FVKSTABINS is specific to the A, B and C phase stabs. FVKSTABINS4 is for the 4th-wire neutral stab only. A 4-wire FVK plug therefore uses both parts. Both are listed for FVK plugs from 30A up to 800A.
General Electric Spectra: one insert for every position
SBCAP is described as a stab insulator insert for the A, B, C and neutral stabs of Spectra Series type SB plugs and BRAH BEB plugs.
Siemens / ITE BD Series: two sizes, two hole positions
For type BOS plugs the insulator changes with the plug amperage: BOSSTABINS1 and BOSSTABINS1H suit 30A, 60A, 100A and 200A plugs, and BOSSTABINS2 and BOSSTABINS2H suit 400A and 600A plugs. Within each size there are two styles. On the plain part number the mounting hole is offset from the notch cut-outs. On the H version the hole is in line with them. Check the hole position on the old insulator against the two descriptions before ordering. The BD Series parts are listed as substitutes for plugs badged Siemens, ITE, Gould and Bulldog.
For anything not in the table, call (855) 355-2724 with the part number.
Why stab insulators fail
- Heat from the stab. A stab with weak spring pressure or a pitted contact face makes a high-resistance joint. The joint heats the plastic around it every time the plug carries load.
- Mechanical damage. Insulators are knocked when a plug is lifted onto or off the busway, or when a plug is forced into an opening it is not aligned with.
- Contamination. Conductive dust, oil mist and moisture settle on the surface and give leakage current a path. Tracking starts there.
- Age and vibration. Plastic that has been through many years of heating and cooling becomes brittle and cracks at the mounting hole or the notches.
- A fault on the plug. An arc near the stabs leaves soot and burned surfaces even where the housing looks whole.
How to inspect stab insulators
Insulators can only be examined with the plug off the busway, so the check belongs in planned maintenance or any time a plug is removed or relocated.
- Look at every face of each insulator in good light, including the side that faces the busway.
- Check around the mounting hole and the notch cut-outs, where cracks usually start.
- Compare the color of the three phases. One darker insulator points to one hot stab.
- Try each stab by hand for movement in its housing.
- Look at the stab itself: pitting, blueing or lost tension means the stab is replaced as well.
- Look at the busway opening the plug came from for matching heat marks.
If the plug has been running, heat marks at the stab end of the plug or on the busway cover near the plug are a reason to schedule a shutdown and take the plug off for inspection.
Replace the insulator with the stabs, or on its own
If the insulator is damaged by impact and the stab is clean and tight, the insulator alone is enough. If the insulator is heat damaged, the stab caused it and both are replaced. The bus plug stabs sub-category holds the matching stabs and kits.
| Plug family and position | Stab insulator | Matching BRAH stabs (catalogue amperage) | How the stabs are supplied |
|---|---|---|---|
| GE Type FVK, phases | FVKSTABINS | FVKSTAB100A (30A – 100A), FVKSTAB200A (200A) | Single stab fingers; insulator ordered separately |
| GE Type FVK, 4-wire neutral | FVKSTABINS4 | FVKSTAB100A4 (30A – 100A), FVKSTAB200A4 (200A) | Single stab fingers; insulator ordered separately |
| GE Spectra type SB | SBCAP | ACSBSLN1 (30A – 200A), 400ASTAB (400A – 600A) | Single stab fingers for any phase including neutral; insulator ordered separately |
| Siemens / ITE BD Series type BOS | BOSSTABINS1, BOSSTABINS1H, BOSSTABINS2, BOSSTABINS2H | BOSSTABKIT1 (30A – 100A), BOSSTABKIT2 (200A) | Kits: complete set of 3 stabs with the associated stab insulators |
| Siemens / ITE XL-U Series type UV | Supplied in the kit | UVSTABKIT1 (30A – 100A), UVSTABKIT2 (200A) | Kits: complete set of 3 stabs with the associated stab insulators |
Choosing the stab itself, including how many are needed per phase, is covered in the guide on identifying the correct bus stabs.
What to check before you order
- Plug type from the nameplate: FVK, SB or BOS for the insulators in the table.
- Plug amperage. For BOS plugs it decides between the BOSSTABINS1 and BOSSTABINS2 sizes.
- Stab position. On FVK plugs the neutral insulator is a different part from the phase insulator.
- Mounting hole position on BOS insulators: offset, or in line with the notches (H version).
- Quantity. Insulators are sold one at a time, so count the positions on the plug: three phases, plus the neutral on a 4-wire plug.
- Whether stabs are needed too. They are separate parts for the GE families and complete kits for BOS and UV plugs.
BRAH Electric designs its parts in-house and is an aftermarket manufacturer, not an authorized distributor of GE or Siemens products. Parts carry a 2-year warranty from the date of invoice. Returns are accepted within 30 days of shipment with an RMA and are subject to a restocking charge, so confirm the style before ordering. In-stock orders placed before 3:00 PM Pacific ship the same business day. Other plug parts are under bus plugs and parts.
Safety when working on stab insulators
The stabs are the part of the plug that touches live bus bars. Take a plug off the busway only with the busway de-energized, locked out and tagged out, and after testing for absence of voltage. Do not put a plug with a damaged insulator back on the busway to keep a line running. Removal, repair and refitting are work for qualified electrical personnel.
Stab insulator FAQs
What is a bus plug stab insulator?
It is the molded plastic housing that surrounds a bus plug stab. It insulates the stab from the other phases and holds it in position for the connection to the bus bar.
Can a cracked stab insulator be repaired?
No. A crack or a carbon track is a permanent loss of insulation. Replace the insulator, and find out what caused the damage before the plug goes back into service.
Does the insulator come with the copper stab?
The seven insulators in the table are sold individually as the plastic housing only, with the stabs sold separately. The BOSSTABKIT and UVSTABKIT stab kits are described as complete sets of three stabs with the associated insulators.
What is the difference between BOSSTABINS1 and BOSSTABINS1H?
The position of the mounting hole. On BOSSTABINS1 it is offset from the notch cut-outs. On BOSSTABINS1H it is in line with them. Both suit BD Series type BOS plugs from 30A to 200A. BOSSTABINS2 and BOSSTABINS2H differ in the same way for 400A and 600A plugs.
Should all the insulators on a plug be replaced together?
When one is heat damaged or brittle with age, the others have seen the same conditions and are usually close behind. Replacing the set while the plug is off the busway avoids a second shutdown.
References
- UL 857, Busways: the UL standard for busways and associated fittings, including plug-in devices.
- NFPA 70B: standard for electrical equipment maintenance, covering inspection and maintenance programs.
- OSHA 29 CFR 1910.147: the control of hazardous energy (lockout/tagout).
- OSHA 29 CFR 1910.333: selection and use of electrical work practices, including de-energizing and verification.
