Frequently Asked Questions

Do Carts Need to be Grounded?

• ​It depends:
  • They do NOT need to be grounded if:
    • The product placed on the shelves is NOT susceptible to damage from ESD
      
      
    • The product is ESD Sensitive but will ALWAYS be stored in a sealed ESD Bag, ESD Tote, or some other acceptable method to protect it from from risks from ESD damage, and as long as the ESD protective container will NOT be opened unless it is moved to an ESD-safe area before removing it from the protective packaging
      
      
• Explanation:
  • If ESD sensitive components will be handled at the cart or shelf outside of their ESD protective enclosure, it should be treated (and grounded) just like an ESD-safe workstation.
    
    

ESD Floor, does it really matter if a few areas fail?

• The ESD Floor (or conductive floor) is typically the cornerstone of an ESD program.  A good conductive floor provides a dependable ESD ground, allowing people, carts, and product to move around an area, increasing mobility and productivity with low risk of damage from Electro Static Discharge.
• If your customers had a choice of having their product built on a part of the ESD Floor that passed or failed, which would they choose?  How would you choose if your job or perhaps your life depended on the quality of the product?

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How should I ground my ESD carts?  Grounding chain / cable, or Conductive Wheel?  (see Demonstration Video)

• The nice thing about grounding chains is that they are cheap, but the down side is that they have a higher failure rate.  Grounding chains and grounding cables are typically lightweight and tend to lose connection with the floor when they run over the smallest "dust bunny"
• Another problem that I see with both the chains and cables is that they work well when they are new and used on an ESD wax or other homogeneous floor, but when they are used on conductive floors with discrete conductive elements such as conductive vinyl floors that use vertical carbon strands, they lose connection frequently.  Conductive vinyl floor tiles are only conductive wherever these vertical conductive elements come to the surface, but the rest of the surface is typically insulative.  If a link of a chain (or ball of a cable) happens to stop on a conductive element, the chain or ball may make contact if the link is clean and heavy enough to make contact.
• In many cases a cart seems to be grounded when it is moving but fails about 50% or more of the times when it stops.  If you are relying on the cart's ground to protect your product, then you typically have less than a 50% possibility of the ground working.
• Conductive wheels are consistently conductive while moving or stopped (assuming the floor is conductive and the wheel is relatively clean) and provide a much more consistent ground than cables or chains.

Will a current limiting resistor keep components safe from quick discharges?

• A current limiting resistor is often built into wrist strap cords, foot straps, and other grounding wires used for ESD purposes.  As usual, the answer is not that simple.  It is often a good idea to have the 1 or 2 mega-ohm resistor for electrical safety.  For example, if you were working on a high voltage device, such as 120 Volts AC, and you got between the 120 Volts AC and the grounded metal work surface, you might very grateful (and alive) if you had a 1 mega-ohm resistor in-between you and the electrical ground.  The same is true with floors, and it may be against the fire code to have a floor that is too conductive due to the increased potential danger from electrical shock.  Floors typically have a direct connection to the electrical or building ground, but there is some resistance built into the floor material.  The resistor keeps you safer when working with higher voltages, but what if you are working with ESD sensitive components?​​​
• An ESD sensitive component can be damaged by placing it on a metal surface regardless of whether it has a current limiting resistor between the cart and ground. 

 

• Example of How a Metal Cart can Damage an ESD Sensitive Part
  • A cart is grounded to earth or electrical ground (with or without a current limiting resistor):  If a component is charged and then placed on the cart, the part will cause an ESD event because the part will discharge to the cart.  The reason for this is that the metal cart will act like a huge capacitor and the discharge from the part will essentially try to charge this big capacitor.  Since the resistor is between the cart and ground rather than between the component and the cart, it will not slow the discharge, because the discharge of the component will attempt to charge the cart. 
  • How quickly will the discharge happen?  Here are the calculations:
  • Using a typical two shelf 36" x  18" wire cart as an example, the capacitance of a cart was measured at 150 nanofarads.   Assuming that the part you placed on the cart is a circuit board and it has a component with metal parts and maybe 0.5 ohm of resistance between the metal and wherever majority of the charge is stored on the circuit board.
  • Calculate the time constant:  R x C = Time Constant
  •           0.5 Ohms * 0.000000150 farads = .000000075 seconds, or 75 nanoseconds
  •           One time constant is the time it takes to discharge 63% of a charge, so...
  • If a device charged to 10,000 volts was placed on a metal cart, it would discharge 6,300 V (63%) in 75 nanoseconds 
    • Current flow would be I=E/R, which is 6,300 V / 0.5 Ohm = 12,600 Amps
    • Power = Volts * Amps, so it would be 6,300 Volts * 12,600 Amps = 79,300 Watts
    • Joules = Watts * Seconds, so 79,300 Watts * 75 nanoseconds = 5.95 Joules
  • Since many components can be damaged by micro-joules, 5.75 joules, could cause considerable damage