Main fuse or main circuit breaker in an electrical panel:

This article summarizes inspection of the main electrical switch - the main switch at buildings and also outlines other electrical panel and switch defects that can be found by visual inspection.

We include information about How many main disconnect switches are allowed, other main disconnect switch hazards.

Inspecting the Residential Electrical Main Disconnect Panel or "Box" for Defects

These questions & answers about the main switch used to turn off electrical power at a building were posted originally at MAIN ELECTRICAL DISCONNECT - be sure to read that article too.

On 2020-07-10 - by (mod) - mounting computer data storage cabinet close to electrical panel risks data loss

John

Thank you for the interesting and helpful question

I am guessing that you're asking about mounting a cabinet used to store computer-data devices such as hard drives or older magnetic-recorded media above an electric main disconnect (typically 240VAC and amperage in the 100 -200 range).

In my OPINION that's a serious risk of data loss.

I recommend keeping a computer data storage cabinet 10 ft. or more from an electrical service, meter, main switch, panel, or service entry cable.

Paraphrasing:

Magnetic fields (including EMF) can cause disturbances in computers, computer storage devices, and in some medical equipment. - (Patrick 2009 p. 436)

The field generated by an electrical service, meter, or around an electrical panel is an EMF or Electromagnetic field - discussed separately

at
DEFINITIONS of EMF RF FIELD & FREQUENCY
and

at EMF ELECTROMAGNETIC FIELDS & HUMAN EXPOSURE

An electrical field can affect magneticall-recorded media. EMP or electromagnetic pulse weapons used to try to damage or destroy an enemy's data or electronic equipment are based on the principle of creating a strong electromagnetic field pulse close-enough to the target equipment to damage it.

Data transmission and storage devices can be designed to be immune to EMF or EMP - typically by using optical data transmission (fibre-optic) and optical data storage devices or laser recording devices.

The good news is that EMF strength falls off as the square of the distance from the source. So you don't need to move too far away to mount your safe data storage cabinet. In the articles I recommended above - the start of our series of articles on EMF and EMR - you'll see that detecting and measuring EMF is pretty easy and inexpensive.

Patent & other research gives some interesting insight into this problem:

• Challener, David C., and Barry D. Atkins. "Electromagnetic field tamper alarm." U.S. Patent 6,553,497, issued April 22, 2003.
  Abstract excerpt:
  The present invention is a method and system for providing a tamper alarm for a computer system. The method includes detecting tampering of the computer system, and transmitting electromagnetic field signals from a device concerning the tampering.
• Churyumov, Gennadiy, Vladimir Tokarev, Vitalii Tkachov, and Stanislav Partyka. "Scenario of interaction of the mobile technical objects in the process of transmission of data streams in conditions of impacting the powerful electromagnetic field." In 2018 IEEE second international conference on Data Stream Mining & Processing (DSMP), pp. 183-186. IEEE, 2018.
• Patrick, Dale R., and Stephen W. Fardo. Electrical distribution systems. The Fairmont Press, Inc., 2009.
• Van Der Togt, Remko, Erik Jan van Lieshout, Reinout Hensbroek, Euro Beinat, Jan M. Binnekade, and P. J. M. Bakker. "Electromagnetic interference from radio frequency identification inducing potentially hazardous incidents in critical care medical equipment." Jama 299, no. 24 (2008): 2884-2890.
  Excerpt:
  Conclusions In a controlled nonclinical setting, RFID induced potentially hazardous incidents in medical devices. Implementation of RFID in the critical care environment should require on-site EMI tests and updates of international standards.

Other types of electrical fields are a greater concern for computer data. Example:

• Palisek, Libor, and Lubos Suchy. "High power microwave effects on computer networks." In 10th International Symposium on Electromagnetic Compatibility, pp. 18-21. IEEE, 2011.
  

On 2020-07-10 by John - Can you mount a data cabinet above a main disconnect box?

Can you mount a data cabinet above a main disconnect box?

On 2020-06-14 - by (mod) - sparking and popping molded main circuit breaker

Joel

Circuit breakers vary a bit in how they work internally, using a thermal, magnetic or both trip mechanisms when there's an overcurrent.

Watch out: The main breaker is really critical, both as a safety device to turn off all electrical power into a building (in an emergency I might not want to stand there scratching my head wondering which breaker controls the circuit where the toaster is electrocuting my friend Paul), and also as a device that will trip and turn off all power in the event of a short circuit that's not caught by the smaller-amperage individual circuit breakers in the panel.

An old breaker may fail mechanically by repeated use (they're not intended to be used as a regular on-off switch), or by corrosion, or even by prolonged heating, and occasionally (you don't give your breaker brand) there by specific breaker design defects that make breakers unsafe and unreliable - those are discussed at this website too, such as FPE, Challenger, Bulldog Pushmatic and some others.

For just switching all power on or off a few times a year, the main breaker should be fine.

Watch out: a sparking, popping circuit breaker has already warned you that your electrical system is very dangerous, at risk of a building fire or a shock hazard that can kill someone. It's possible as well, that the breaker's arcing has already damaged the main bus to which it connects or to other panel parts such that the panel needs to be replaced.

Your breaker may now fail to trip in response to an overcurrent - inviting a fire, injury, or death.

It and perhaps other parts need to be replaced. In the meantime i would NOT leave power ON at this building.

On 2020-06-12 by joel herzog

I have who inherited home with a sparking and popping molded main breaker (used over years to shut down off season) I can't find electrician they have confidence in because none give straight answers why and what older molded breaker does and what is suitable replacement, such as replace and upgrade entire panel and what is feasible main to use as disconnect method in off season

On 2019-09-17 - by (mod) - Do I need a form of disconnect at the meter if the panel has a main breaker?

Alec

Possibly yes. The answer depends on where the main switch and other electrical pane are located as well as the number of breakers in the panel.

In a typical residential electrical installation at which the electric meter is mounted on the building exterior and a short service entry wire brings power in to an electrical main panel close by, a separate disconnect is not provided at the meter. Instead, if the electric company or an electrician needed to drop power into the home (in an emergency for example) they's just pull off the meter.

However when the electric meter is some distance from the building it serves, it's good practice to install a main breaker or fuse and thus a disconnect at the meter. That additional switch and breaker or fuse will protect the wire between the meter and the building. You'd still want the main disconnect in the building too.

On 2019-09-17 by Alec B

Do I need a form of disconnect at the meter if the panel has a main breaker?

On 2019-03-25 by (mod) - rule of six - breakers before a main switch is needed

A common rule of thumb is 6 breakers max to be switched off to cut all power to a home; more than 6 you need a separate main switch.

On 2019-03-25 by Anonymous

How many disconnects are allowed on a residential home to turn off

On 2018-11-17 by Basil

HV Transformer supplying a 1000 AMM SCR Panel. Main Disconnect installed ahead of the Transformer. All together in the Tx vault room. Do I need an additional disconnect between ahead of the SCR panel on the load side of the the Transformer or will the Main Disconnect ahead of the Transformer suffice? Please reference NEC with reply.

OOPs! Should be 1000AMPs,

On 2017-11-29 by (mod) - the main On-Off switch can be in the main electrical panel or it can be a separate switch.

Diana

Yes the main can be in the main electrical panel or it can be a separate switch.

Also any panel with 6 or more circuits needs a cutoff switch.

On 2017-11-29 by Diana

Is it code to have a main shut off in the breaker box?

by (mod) -

John, we give details of service entry cable wiring sizes at SE CABLE & BRANCH CIRCUIT WIRE SIZES vs AMPS

found by using the search box above to look for "SERVICE ENTRY WIRE SIZE" - please take a look at that article and if you have more questions do not hesitate to ask me.

On 2017-08-13 by John

I am updating old house electrical. My ? Is what size alm. Wire do I need for a 200 Amp meatier base then without conduit intearer panel is 20' from meatier base going under house up to the panel 100 Amp panel what size alm.wire do I need?

On 2017-05-26 by (mod) -

Mark,

It depends.

Since a new home is bound to have more than 6 electrical circuits, certainly a main shut off switch is required. However your local Electrical Codes May indeed permit that main switch to be physically separate from although next to or very close to the main panel.

Take a look and tell me what you see

On 2017-05-25 by Mark S

we are having a new home built in Gulf Shores Alabama. The electrical is required to meet NEC 2014. The circuit breaker box has no main shut off. this to code?

On 2017-05-04 by Anthony Naylor

Electrical main disconnect On permanently locking it in the on postion

What is the code governing this

On 2017-01-05 by (mod) - Main Disconnect switch for a house be located outside the Power Company meters?

Jose I perhaps don't understand the question. An electric meter is typically a glass/plastic enclosed metering system that does not include main disconnects.

However it is common to see a common control box that combines a meter with a main shutoff - at least in some jurisdictions. You'll see photos of them in this article series.

The main disconnect can also be inside as a separate switch or in the main panel. There are rules about wiring between the meter and the load center or switch since in that case it's not fused.

See the U.S. NEC articles 422 - 450.
Here is a key excerpt paraphrasing from our much-admired expert Mike Holt:

Install the service disconnecting means at a readily accessible location.

This can be outside the structure. If you place it inside the structure, locate it nearest the point of service conductor entry. There's another factor to consider when locating a disconnecting means indoors — you can run the service entrance conductors only so far.

These conductors do not have short-circuit or ground-fault protection.

Thus, they must be limited in length when inside a building. Some local jurisdictions have a specific requirement as to the maximum length within a building.

If yours doesn't have a specific requirement, get the approval of the AHJ before finalizing the location.http://ecmweb.com/code-basics/article-230-services-part-1

On 2017-01-05 by Jose

Trying again. Does the main breaker disconect switch (only) has to be located next to the power company meters outside the house> or can it be inside with all the other breakers?

I meant next to the meters

Does the NAC require that the Main Disconnect switch for a house be located outside the Power Company meters?

On 2016-03-29 by eric

what is the requirments for the location of the main disconnect on machine power

On 2015-10-18 by shermly simon

Need more information on such topics.

...

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