# Surge protection for intercom

#### ronschuster

Joined Jun 11, 2012
2
I'm replacing my whole-house wired intercom system with a home-brew system I'm currently designing. The stations are connected to the master station using 6-wire (3 twisted pairs) cable. I will be using one pair for power, one for data, and one for audio. The question I am currently dealing with is, what should I use to protect the circuitry from large voltage spikes caused by near-by lightning strikes? I recently had an experience that showed me first-hand how large these can be. I had pulled a pair of wires out from behind the cabinet of one of the existing old stations to test something and they were currently not connected to anything. They were just hanging there, bare ends maybe about 1/8 to 1/4 inch apart. During a thunderstorm we had a near-by lightning strike and I saw a spark jump that gap. I was amazed, though I don't know if I should have been. This is an area I have no experience in. Searching online for surge protection I get info for MOV, TVS, TSPD, etc, etc. I'm not sure where to start. I understand some of these can wear out over time. Can someone give me a recommendation? Keep in mind I am doing this on a hobbyist budget.

#### wayneh

Joined Sep 9, 2010
17,153
To my knowledge, my home intercom has no such protection. My point is, a real solution may cost more than the device being protected. I'm not sure there's a lot that can realistically be done to protect against a lightning strike. They do sell telephone-line and cable TV "surge" protectors, protecting more than just your power lines. Maybe they help.

The spark you observed may have been high voltage but very low current, like a static discharge. Static can be tough on electronics but good grounding can deal with a lot of that.

#### ronschuster

Joined Jun 11, 2012
2
Just to be clear, I don't expect to be able to protect against a direct hit, but I thought some kind of surge protection would be appropriate. Like, would just throwing some MOVs across the lines be helpful, or is that just too simple-minded or old-school?

#### mcasale

Joined Jul 18, 2011
210
In the past I've used zener diodes across digital lines for ESD protection. Nowadays, people tend to use TVS diodes because of lower capacitance and faster signal speeds (like USB).

For general protection, I'd just buy a bunch of plug-in surge suppressors to protect the AC power lines. The more, the better. Keep in mind that if there's a big enough surge, these will be damaged and have to be replaced.

#### #12

Joined Nov 30, 2010
18,222
I consider a "whole house" lightning protector to be essential. $35 at Home Despot the last time I looked. You install it on the main breaker panel. That catches the big jolts and the little 6 outlet strips with MOV's inside provide added protection at the place where you plug in your electronics. #### wayneh Joined Sep 9, 2010 17,153 But aren't the intercom wires themselves - not the power lines - the threat that the OP is concerned about? Like plumbing, they traverse the house and provide a path to ground. #### #12 Joined Nov 30, 2010 18,222 If one prefers not to protect the most likely entry point for lightning surges, that is OK with me. #### wayneh Joined Sep 9, 2010 17,153 I took for granted - assumed - he already knows about surge protectors and such for the power supply, and was asking about the in-house lines. #### westom Joined Nov 25, 2009 52 Searching online for surge protection I get info for MOV, TVS, TSPD, etc, etc. I'm not sure where to start. Start with what was known hundreds of years ago. Lightning is an electrical current from cloud to earthborne charges. It takes the electrically shortest path. That is not 5 miles across the sky and down to earth. That is 3 miles down to earth and four miles through earth to those charges. A better electrical conductor connects lightning to earth - a wooden church steeple. But wood is not an ideal conductor for those 20,000 amps. A high voltage is created. 20,000 amps times a high voltage is high energy. Church steeple destroyed. Franklin installed a lightning rod. Does the lightning rod do protection? Of course not. A better electrical conductor connects to what does protection - earth. The better conductor to earth means 20,000 amps creates a near zero voltage. 20,000 amps times a near zero voltage is near zero energy. No damage. You have the exact same problem. You saw a spark because lightning found a best path to those earthborne charges (miles away) via the wire that sparked. You have all but invited lightning to be inside hunting for earth destructively via appliances. All protection systems have one critically important item. The connection to earth. Some have no protectors. But every protection system has the only item that absorbs hundreds of thousands of joules. Earth ground. How many joules on those MOVs, etc? Even 100 years ago, well proven protection systems connected hundreds of thousands of joules harmlessly to earth. And no damage. But that means the electric current must not use your appliances or a wooden church steeple as the connection. Lightning striking AC wires far down the street is a direct strike to every household appliance. Is every appliance damaged? Of course not. It is called electricity. Every appliance has an incoming path. But only the damaged one also had an outgoing path to earth. So that the surge is not inside, you must earth every wire inside every incoming cable. Then lightning need not hunt for earth destructively inside. For example, code says the cable TV wire must connect to earth before entering. That connection (and no protector) is the best protection. Then lightning need not be inside hunting for earth via cabled appliances. Telephone will not work if earthed directly. So all telephone lines already have a 'whole house' protection installed for free. What makes it effective? At this point you should know that answer. *Earthing*. It must connect low impedance (ie 'less than 10 feet') to your building's single point earth ground. Same ground also used by TV cable and AC electric. Due to so many who never learned this stuff (and still make recommendations), then most every homeowner does not have a 'whole house' protector on AC mains. These protectors are from companies with superior reputations including ABB, Siemens, Square D, Leviton, Polyphaser, General Electric, and Intermatic. A Cutler-Hammer protector sells in Lowes and Home Depot for less than$50. Superior protection, even from direct lightning strikes, costs that little. However and again, every protector is only as effective as the item that does all protection. Single point earth ground.

Welcome to how it was done even 100 years ago. Notice how many want protection in terms of a magic box rather than learn hundreds of years of well proven science. You had a spark because you all but invited lightning to be hunting inside. Only superior protection already inside every appliance has been averting repeat damage.

You should have reams of questions. And not about the simple science - a protector. You should be asking extensively about the only item that defines all protection - single point earth ground. The 'art' of protection. And the item most forgotten because they do not see it.

No earth ground means no protection. A protector is only as effective as its earth ground. Hundreds of thousands of joules (and smaller surges) must dissipate somewhere. That ‘somewhere’ defines protection. Franklin connected ligthning to earth on a path that remained outside the church. Therefore no more church damage. You must do same even for household appliances. A 'whole house' protector connected to earthing that both meets and exceeds code reqirements.

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#### wayneh

Joined Sep 9, 2010
17,153
No earth ground means no protection.
Not entirely; no earth ground can also mean no risk. You're reasonably safe from a strike if you're not grounded. The problem is when you, or something you care about, completes the path to a nearby ground.

But that's a small quibble to this interesting post.

In the context of the OP's problem, does this mean that dedicating one or more of the system's conductors to an earth ground line would offer significant protection? Sounds better than nothing, but I'm not sure it would offer real protection.

#### westom

Joined Nov 25, 2009
52
In the context of the OP's problem, does this mean that dedicating one or more of the system's conductors to an earth ground line would offer significant protection?
Previously posted
So that the surge is not inside [a building], you must earth every wire inside every incoming cable. Then lightning need not hunt for earth destructively inside. For example, code says the cable TV wire must connect to earth before entering. That connection (and no protector) is the best protection. Then lightning need not be inside hunting for earth via cabled appliances.
Don't earth the victim. Earth a surge.

This is the only solution implemented in every facility that cannot have damage. Your telco CO connects all over town. Is confronted by over 100 surges with each storm. Do they disconnect all phone service with each storm? Does you town have no phone service for four days after a storm? Of course not. Because every incoming wire to the \$multi-million computer is earthed BEFORE entering the building.

Protectors that make that earth connection are located up to 50 meters distant from the computer. Increased separation between protector and electronics increases protection. But more important is the 'as short as possible' distance to single point earth ground.

#### bud--

Joined Jun 13, 2012
15
The best information on surges and surge protection I have seen is at:
http://www.lightningsafety.com/nlsi_lhm/IEEE_Guide.pdf
- "How to protect your house and its contents from lightning: IEEE guide for surge protection of equipment connected to AC power and communication circuits" published by the IEEE in 2005 (the IEEE is a major organization of electrical and electronic engineers).
And also:
http://www.nist.gov/public_affairs/practiceguides/surgesfnl.pdf
- "NIST recommended practice guide: Surges Happen!: how to protect the appliances in your home" published by the US National Institute of Standards and Technology in 2001

The IEEE surge guide is more technical.

The guides have limited usefulness for the question the OP asked.

Franklin installed a lightning rod. Does the lightning rod do protection? Of course not. A better electrical conductor connects to what does protection - earth. The better conductor to earth means 20,000 amps creates a near zero voltage. 20,000 amps times a near zero voltage is near zero energy. No damage.
Near zero voltage?

Suppose you have a 1000A surge to the earthing system at a house and the only electrode is a ground rod with a near miraculous 10 ohms resistance to earth. The building ground system will rise 10,000V above 'absolute' earth potential. In general 70% of the voltage drop away from the rod is in the first 3 ft. The building 'ground' will be at least 7,000V to earth over 3 ft from the rod.

For lightning rods also consider the voltage drop on the down conductors. Lightning is a very short event. That means it has relatively high frequency current components. That means the inductance of the wire is more important than the resistance. For a lightning protection system, metal within 6 feet of the rods and conductors may have to be bonded to the protection system. An example might be a roof top heating/cooling unit. It has to be bonded because the voltage on the lightning protection system on the roof may be high enough to produce a side flash (6 feet) to the unit.

In a house, phone and cable (and dish) wires entering the house have entry protectors that are connected with short ground wires to the power earthing system. During an 'event' the ground system may rise far above 'absolute' earth potential (as above). Much of the protection is that all wiring rises together. If the connecting ground wires are not short enough there may be damaging high voltage between power and signal wires. This is illustrated in an example in the IEEE surge guide starting page 30.

If a strong surge comes in on power wires, at about 6,000V from service panel busbars to the enclosure there is arc-over. When the arc stabilizes the voltage is hundreds of volts. Since the enclosure is connected to the earthing system this dumps most of the surge energy to earth.

You have the exact same problem. You saw a spark because lightning found a best path to those earthborne charges (miles away) via the wire that sparked. You have all but invited lightning to be inside hunting for earth destructively via appliances.
The wires were not connected on one end. From the information given the spark was likely direct pickup.

So that the surge is not inside, you must earth every wire inside every incoming cable. Then lightning need not hunt for earth destructively inside. For example, code says the cable TV wire must connect to earth before entering. That connection (and no protector) is the best protection.

No protector is the best protection?

The code just requires a ground block that allows the coax shield to be earthed. The IEEE surge guide says there is no requirement to limit the voltage developed between the core and the sheath. .... The only voltage limit is the breakdown of the F connectors, typically ~24 kV. And "there is obviously the possibility of damage to TV tuners and cable modems from the very high voltages that can be developed, especially from nearby lightning."

Due to so many who never learned this stuff (and still make recommendations), then most every homeowner does not have a 'whole house' protector on AC mains. These protectors are from companies with superior reputations including ABB, Siemens, Square D, Leviton, Polyphaser, General Electric, and Intermatic.

Service panel protectors are a real good idea.
But from the NIST guide:
"Q - Will a surge protector installed at the service entrance be sufficient for the whole house?
A - There are two answers to than question: Yes for one-link appliances [electronic equipment], No for two-link appliances [equipment connected to power AND phone or cable or....]. Since most homes today have some kind of two-link appliances, the prudent answer to the question would be NO - but that does not mean that a surge protector installed at the service entrance is useless."

Service panel protectors are very likely to protect anything connected only to power wires from a very near very strong lightning strike.

Service panel suppressors do not by themselves prevent high voltages from developing between power and phone/cable wires. The NIST surge guide suggests most equipment damage is from high voltage between power and signal wires.

Notice how many want protection in terms of a magic box rather than learn hundreds of years of well proven science.

The "magic box" is a plug-in protector. It is only magic for westom.

The IEEE surge guide explains how plug-in protectors work starting page 30. It is not primarily by earthing the surge because the impedance of the ground wire is too high. They work primarily by limiting the voltage from each conductor (power and signal) to the ground at the protector. The voltage between wires going the protected equipment is safe for the protected equipment.

Almost all the companies above with "superior reputations" make plug-in protectors.

When using a plug-in protector all interconnected equipment needs to be connected to the same protector. External connections, like cable, also must go through the protector. Connecting all wiring through the protector prevents damaging voltages between power and signal wires. This is a problem for the wired intercom.

Voltage limiters like zeners or TVS diodes (as in the post from mcasale) from signal wires to circuit ground might be the best protection. MOVs may work if the capacitance is not too high.

No earth ground means no protection. A protector is only as effective as its earth ground.
Plug-in protectors do not work by earthing. Both the IEEE and NIST surge guides say they are effective.

#### wayneh

Joined Sep 9, 2010
17,153
Perhaps the best first post ever!

#### westom

Joined Nov 25, 2009
52
Perhaps the best first post ever!

His own citations contradict his claims. He quotes out of context and without numbers: For example, NIST on page 17 describes his products as "useless".
A very important point to keep in mind is that your surge protector will work by diverting the surges to ground. The best surge protection in the world can be useless if grounding is not done properly.
No way around that reality proven by over 100 years of experience and science.

If accurate or honest, then he included manufacturer spec numbers that claim that protection. For almost a decade, he never does. Sometimes claiming he need not post spec numbers because any child can read them. He really means those spec numbers do not exist. Since his protectors only claim near zero protection.

His IEEE citation shows what happens when a protector is too close to appliances and too far from earth ground. It earths 8000 volts destructively through any nearby appliance. On page 42 figure 8, a power strip protector (with no earth ground) earths 8000 volts destructively through TV2.

How can it earth 8000 volts when he repeatedly claimed surges do not exceed 6000 volts? He also refuses to answer that contradiction. He has long since learned not to post numbers. Sales promoters make subjective claims to promote a "useless" product.

Every professional organization says a low impedance connection to earth must exist to have protection. To keep you confused, he will then talk about flying airplanes - as if that is relevant to building and appliance protection. He is a sales promoter who just cannot find the manufacturer spec numbers for what he recommends.

Protection is always about where hundreds of thousands of joules dissipate. Always. Where are hundreds of thousands of joules absorbed? The protector only claims hundreds of joules. Page 42 Figure 8. it earths 8000 volts destructively when located too far from earth.

Superior solutions that cost less money come from companies that any 'guy' knows for their quality. Many examples listed previously. Effective protectors do what the NIST, IEEE, and other professionals require for protection. That includes a dedicated and always required wire for the low impedance (ie 'less than 10 foot') connection to earth.

All appliances (including the intercom) already contain superior protection. You concern is a rare surge that can overwhelm that protection. Every facility that cannot have damage upgrades the earthing to meet and exceed code. And earths a 'whole house' protector. What is always found on any protector that is not called "useless"? A dedicated wire to single point earth ground. Because the effective solution earths hundreds of thousands of joules - harmlessly.

How surge protection works was defined earlier. Suddenly a magic box will stop what three miles of sky could not? The myth is extremely profitable. Protection means one knows where energy - hundreds of thousands of joules - are absorbed. A protector is only as effective as its earth ground. Insults do not change 100 years of experience and well proven science. A protector is only as effective as its earth ground. As is found in every facility that cannot have damage - including munitions dumps.

How to quickly become unemployed? Install a power strip protector in a munitions dump.

#### bud--

Joined Jun 13, 2012
15
His own citations contradict his claims. He quotes out of context and without numbers: For example, NIST on page 17 describes his products as "useless".

Lie #1.
The NIST, of course, never says plug-in protectors are useless.
What does the NIST surge guide really says about plug-in protectors?
They are "the easiest solution".
And "one effective solution is to have the consumer install" a multiport plug-in suppressor.

With no sources that agree with him westom has to misrepresent what the NIST says.

Lie #2
They are not 'my products'.

If accurate or honest, then he included manufacturer spec numbers that claim that protection. For almost a decade, he never does.

Lie #3, and a favorite
I have posted specs often. Others have posted specs. Westom always ignores them.
Anyone here could find specs (with one exception).

Lie #4.
I first saw westom far less than a decade ago. But I have been told by other people he has been compulsively posting this drivel for that long.

His IEEE citation shows what happens when a protector is too close to appliances and too far from earth ground. It earths 8000 volts destructively through any nearby appliance. On page 42 figure 8, a power strip protector (with no earth ground) earths 8000 volts destructively through TV2.
Anyone with minimal mental abilities can discover what the IEEE guide says in this example:
- A plug-in protector protects the TV connected to it.
- "To protect TV2, a second multiport protector located at TV2 is required."
- The illustration "shows a very common improper use of multiport protectors"
- In the example a surge comes in on a cable service with the ground wire from cable entry ground block to the earthing system at the power service that is far too long. In that case the IEEE guide says "the only effective way of protecting the equipment is to use a multiport [plug-in] protector."
- westom's favored power service protector would provide absolutely NO protection.

It is simply a lie (#5) that the plug-in protector in the IEEE example damages the second TV.

With no sources that agree with him westom has to misrepresent what the IEEE says.

How can it earth 8000 volts when he repeatedly claimed surges do not exceed 6000 volts? He also refuses to answer that contradiction.
Lie #6.
It has been explained but is too complicated for westom.
You don't get over about 6kV on power wires because of arc-over at the panel.
The voltage from the cable entrance protector to the ground at the service panel in the IEEE example is 10kV. It obviously is not limited by arc-over in the service panel.

To keep you confused, he will then talk about flying airplanes - as if that is relevant to building and appliance protection.

Westom's mantra is "a protector is only as effective as its earth ground".
Then do flying airplanes drag an earthing chain? Or are they crashing when they regularly get hit by lightning.

He is a sales promoter who just cannot find the manufacturer spec numbers for what he recommends.
Lie #7
The only association I have with the surge protection industry is I am using some protectors.

Lie #8
I recommend only accurate information. Then people can make appropriate decisions. In fact I suggested that plug-in protectors for wired intercom might not be a good idea.

Protection is always about where hundreds of thousands of joules dissipate. Always. Where are hundreds of thousands of joules absorbed? The protector only claims hundreds of joules.

As explained in my previous post, most of the energy in a strong surge on power service wires is dumped to earth via arc-over from service panel busbars to the enclosure and earthing electrodes.

The author of the NIST surge guide investigated how much energy might be absorbed in a MOV in a plug-in protector. Branch circuits were 10M and longer, and the surge on incoming power wires was up to 10,000A . (That is the maximum that has any reasonable probability of occurring and is based on a 100,000A strike to a utility pole adjacent to the house in typical urban overhead distribution.) The maximum energy at the MOV was a surprisingly small 35 joules. In 13 of 15 cases it was 1 joule or less. The plug-in protector I am using that has the lowest ratings has a total joule rating of 1770 - 90,000 surge amps. Compare that with the 35 joules maximum above. High ratings mean long life. A plug-in protector, wired correctly (as in my first post), is very likely to protect from a very near very strong lightning strike. (The 90,000A is far above what can appear on even the power service wires. The high surge amp rating just goes along with the high joule rating.)

One reason the energy is so small is the arc-over at about 6,000V with arc voltage of hundreds of volts. The other reason is the impedance of the branch circuit at surge current frequencies greatly limits the current, and thus energy, that can reach a plug-in protector.

(Neither service panel or plug-in protectors work by absorbing the surge. But they both absorb some energy in the process of protecting.)

Superior solutions that cost less money come from companies that any 'guy' knows for their quality. Many examples listed previously.

All of westom's favorite companies except SquareD and Polyphaser also make plug-in protectors and say they are effective.

For its "best" service panel protector SquareD says "electronic equipment may need additional protection by installing plug-in [protectors] at the point of use."

All appliances (including the intercom) already contain superior protection.

Some equipment has some surge protection. Some has no protection. None is likely to have as much protection as a plug-in protector.

And earths a 'whole house' protector.

Repeating from the NIST surge guide:
"Q - Will a surge protector installed at the service entrance be sufficient for the whole house?
A - There are two answers to than question: Yes for one-link appliances [electronic equipment], No for two-link appliances [equipment connected to power AND phone or cable or....]. Since most homes today have some kind of two-link appliances, the prudent answer to the question would be NO - but that does not mean that a surge protector installed at the service entrance is useless."

A protector is only as effective as its earth ground.

Westom's mantra protects him from conflicting thoughts (aka reality).

Everyone is in favor of earthing the building ground system. Unfortunately for westom the IEEE surge guide explains (starting page 30) that plug-in protectors do not work primarily by earthing - earthing occurs elsewhere in the system. Plug-in protectors work by limiting the voltage from each wire (power and signal) to the ground at the protector.

For real science read the IEEE and NIST surge guides. Excellent information on much more than has appeared here. And both say plug-in protectors are effective.

Then read the sources that agree with westom that plug-in protectors do NOT work. There are none.

#### wayneh

Joined Sep 9, 2010
17,153
I believe the OP is still looking for a recommendation of what to do with his DATA lines, not just the power lines to his intercom.

FWIW, the intercom and security wiring in my house has no special protection. The whole intercom system is powered by a standard wall-wart plugged directly into the house's AC power, which is likewise unprotected by any add-on device. Surfing nude, if you will. Keep the insurance paid up and forget about it.

#### westom

Joined Nov 25, 2009
52
I believe the OP is still looking for a recommendation of what to do with his DATA lines, not just the power lines to his intercom.
Every recommendation defined protection of data lines. Those who do this stuff know that increased distance between a protector and electronics INCREASES that protection. In any facility that protects data lines, a protector is earthed where wires enter the building. If a surge is permitted inside, then nothing can avert that destructive hunt for earth.

Any protection that works at an intercom is already inside the intercom. Even 1970 design standards defined it. Today's design standards demand even better. OP's concern is a rare surge that can overwhelm that protection. Solution means no transient currents hunting inside a building. Then protection inside every appliance (including the intercom) is not overwhelmed.

A semiconductor by itself may be destroyed by only 40 volts. That same semiconductor, when part of a system, can withstand hundreds or thousands of volts without damage. Superior protection is routine inside every appliance.

#### bud--

Joined Jun 13, 2012
15
I believe the OP is still looking for a recommendation of what to do with his DATA lines, not just the power lines to his intercom.
Mcasale suggested zener or TVS diodes. I suggested those plus maybe MOVs.

Wall-warts (as in wayneh's system) seem to be fairly immune to surges.

If 120V power wires connect to the intercom I would wire a MOV H-N but not to ground. MOVs on power wires should be protected from failure. They fail by starting to conduct on normal voltages and going into thermal runaway.

If a surge is permitted inside, then nothing can avert that destructive hunt for earth.
Both the IEEE and NIST surge guides say plug-in protectors are effective.
But all signal wires should go through them - they are not a good choice for a wired intercom.

The NIST surge guide says (page 14):
"Intruder alarm systems using wires between sensors and their central control unit can be disturbed - and damaged in severe cases - by lightning striking close to the house. The wires necessary for this type of installation extend to all points of the house and act as an antenna system that collects energy from the field generated by the lightning strike, and protection should be included in the design of the system, rather than added later by the owner."

The signal wires in wayneh's systems may or may not be protected by the manufacturer.

Near lightning strikes can directly induce voltages with wires acting as "long wire" or "loop" antennas. A loop could be cable and power wires, with a common connection to the earthing system where the wires enter the building and the loop closing at a TV. Voltage depends on the area of the loop and orientation of the field from the lightning. For an intercom, the loop can be power and intercom wires.

This is not a common source of damage.

My guess is the spark the OP observed was from wires acting as an antenna.

Any protection that works at an intercom is already inside the intercom.

Nonsense. Equipment may or may not have protection.

Irrelevant anyway since the OP wants to build an intercom.

Even 1970 design standards defined it.

What standards?
Enforced by who?