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SURGE: DC and AC 

Technically, a 'surge' is a variation from the controlled steady state level. When applied to a DC system like those in mobile and marine applications, it results in a temporary increase in DC voltage. But, we also need to consider surges on AC power lines, since at one time or other most DC systems will be connected to battery chargers.

 

DC SURGE:

In a properly operating system, the 'permissable' level of surges is 100 Volts for up to 50 ms and 70 volts for 100 ms from a source impedence of 500 milliohms (from MIL-STD-1275B).

This illustrates the danger for equipment connected to mobile and marine electrical systems. Most engineers looks at data supplied by manufacturers of MOV or Surge Suppresssor diodes and are misled into thinking they are capable of protecting dc powered equipment. These devices are designed to suppress remote lightning strikes on power lines. They exhibit an 8/1000 us characteristic - meaning they are designed for surges that reach a peak at 8 us and have decayed by 1000us, or 1 ms.

In dc electrical systems, surges last several hundred times longer! The power ratings of the protection devices are quickly exceeded which causes them to fail - short circuiting the power supply section of the equipment. Excessive current flows causing printed circuit board traces to vaporize (see Reverse Polarity photo). An expensive trip to the repair shop follows.

Ironically, if your equipment has built-in surge protection, it is more likely to be damaged by your dc electrical system! Use a STO-P Power Fault Protector to prevent this from happening.

 

AC SURGE:

In common 'usage', a SURGE has been equated to AC power line variations due to lightning strikes or other faults on the distribution system. These 'AC SURGES' need to be considered in the design of DC power systems, since at some point a mains powered battery charger is likely to be connected. For marine systems, AC battery chargers are routinely used to keep batteries fully charged whenever the vessel is at a dock.

AC Surges are coupled into the DC power supply causing severe fault conditions.

The table below indicates the frequency of surge transients on 120 Volt AC power lines in the United States. The corresponding voltage levels coupled to the DC power system will depend on the design of the charger and the chacteristics of the AC distribution network. But, the table clearly shows that significant surge voltages can be applied quite frequently even in geographical areas considered to be at 'low risk'.


 
Surge  Frequency Voltage Level
Low Risk 50 hits/year 350 V
10 hits/year 500 V
  1 hit/year 920 V
Moderate Risk 50 hits/year 2100 V
10 hits/year 2700 V
  1 hit/year 4900 V

Even in low risk areas , one can expect surge levels at the 'serious' or 'dangerous' level at least several times each year. Since many, if not most, battery chargers DO NOT have any form of output filtering to suppress them, high voltage surges can enter the 12/24/28 volt power bus at unpredictable levels.

STO-P UPS responds to AC Surges that pass through the battery charger in the same manner as Transients - they are detected and removed within the first few nanoseconds. It's important to remember that the only real protection in DC systems is having power 'scrubbed' clean just prior to entering the equipment.

 

Note: If you use standard 'surge suppressors' on AC lines, do so with caution. Most low cost suppressors use MOV's (Metal Oxide Varistors) which have a limited working life. In some cases, a single high voltage surge can render them inoperative! Unfortunately MOV's produce no indication that they are no longer functioning - the user is left with the false impression that his system is protected when the opposite may be true.

 

 

 

         
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