Security Cable Voltage Drop Part 2 Technical Calculations & Solution

February 23rd, 2014

Voltage drop issues in security systems can be avoided with some simple calculations leading to correct cable selection.

We power electric locks and other devices via sheathed copper cable. This cable has a known resistance per meter and the electric device will require a known current.
The resistance causes the voltage to drop as per Ohms law. Naturally the voltage drop will rise as the cable gets longer. The third factor for calculating the voltage drop is the current required. The formula is V=IR (Voltage = Current X Resistance).
Resistance and current values are usually stated in the equipment data sheets and cable data often also comes with a formula that is essentially Ohms law.

Formula and Calculation
Basically V(drop) = I X R
I = Current required by the end device
R = Total resistance for the cable for the whole run

An example is provided below.
Cable distance = 100m X .03 ohms per meter = 3 ohms per 100m
Lock current draw = 1Amp,
V(drop) = 1Amp X 3 Ohms = 3V

A 3V drop from the 13.8V power supply would be unacceptable.

Usually as the copper conductor size increases the resistance will drop. So select a larger cable that will provide a voltage drop within operating voltage tolerances of the device at the end of the cable. Be warned that the resistances between cable manufacturers and types can vary. So it is best to check the cable data prior to using a new brand or type.

Cable is relatively cheap compared to labour, so it is a good practice to rough in a slightly larger cable than you think you will need. This will save a costly re-run if the voltage drop proves too much or the current requirement is changed. It also means you don’t have to worry about the V(drop) calculation on every run, just the long or high current ones.


Spot the problem?

February 21st, 2014

Mag lock mounted wrongMag lock all wrong

Yes that is a sliding door!

This mag lock was installed by a carpenter, it is switched via a cheap hotel style locking system. Obviously this did not work long due to the lock grabbing the armature plate as soon as it slid close to the face. The door would not close and quickly the plate was dislodged. The mag was also packed down with cheap plastic spacers.

An experienced locksmith was called in and paid to install the proper L&Z configuration. The L&Z bracket was supplied with the lock but the original installer obviously did not have a clue.

Thanks to the locksmith (Luke) for the photos. Another reason only qualified and experienced tradesmen should be used.


Security Cable Voltage Drop Part 1 – Explanation and Diagnostics

February 21st, 2014

Cable voltage drop refers to the lower potential available at the far end of a cable run compared to the potential available at the power supply end. Voltage drop in access control systems is often ignored until it presents a problem and can be difficult to diagnose.

The effect of voltage drop is most often encountered with high current devices such as mag locks at the end of a long cable run. The voltage drop will cause a weak bond and will often cause the bond sense circuit to fail.

A voltage drop fault can be diagnosed by checking the voltage at the lock under load and no load conditions. The no load condition should be very close to the voltage at the power supply. Under load a severe problem will cause a drop of 2 or more volts at the lock while the voltage at the power supply will appear relatively unaffected. The voltage is lost in the cable.

Very occasionally cable damage or a manufacturing fault will cause this problem even on short runs but in the vast majority of cases the long cable run cannot support the required voltage at that current.

This problem is not limited to electric locks; any device with an insufficient cable can be affected. Voltage drop faults are often seen at field door controllers, remote devices and external gates.

In part 2 we will talk about solutions to this problem and some related calculations.


Mag Lock Slow to Release?

December 19th, 2013

If you are finding a mag (electromagnetic) lock to be “sticky” or slow to release it is likely that it has a diode connected across the power terminals. Remove it, problem solved.

This is a common mistake. The reverse biased diode is supposed to prevent a current spike (often call back EMF) caused by the decay of the magnetic field around a coil. This is true of basic coils such as those found in relays and unprotected strikes. However the combination of the diode and the internal electronics of modern mag locks combine to cause a slow release of the armature plate.

This is often very noticeable to the door user. In one particular case I came across recently an entire buildings worth of doors had this problem. It was exacerbated by the fact that the push to exit buttons used the wrong contact. The problem was so bad that the security manager had put up signs that read “press button… wait… open door.” The same company that incorrectly installed the diodes and exit buttons had also glossed over the problem for five years of annual maintenance.

Protection diodes should not be installed in modern electromagnetic locks. The delay can cause excessive force to be placed on the lock by users and I have even seen people bump into the door expecting it to open sooner. It is also a major problem when a mag lock is combined with a power swing door operator, the operator fights against the mag for a second shortening the life of both parts.


Australian Standards – Security Systems – Part 2 EOL Resistor Termination

August 17th, 2013

This post partly relates to the ATMOD a Jack Fuse product that houses End of Line (EoL) resistors on a PCB with quick connect terminals.

ATMOD pre-built EOL resistor alarm pack

ATMOD EOL Reistors for Security/Alarm Systems

A potential customer recently scoffed at the ATMOD claiming it did not comply with Australian standards. Apparently this customer was under the impression that the standards dictate that resistors must be connected via soldering and insulated with heat shrink.

In fact the standard does not directly cover the termination method for EOL resistors. AS/NZS 2201.1 does refer to termination methods in general including crimping, soldering, terminals and self locking connectors. The ATMOD uses self locking connectors. All that is needed to make the ATMOD comply with the termination standards is for the installer to use the correct cable size. (Cable types are stated in the ATMOD data.) Security four core is perfect.

To be fair to the potential customer, I subscribed to the solder and heat shrink method of termination for many, many years and still use it on occasion. Apart from the ATMOD I believe soldering to be one of the better/more reliable methods for connecting EOL resistors inline.

axial resistors

Axial Resistors

There is one drawback with soldering, the axial resistors supplied with all alarm panels have solid conductors and are designed to be soldered to a PCB with strain relief. Axial resistors were never designed to be soldered in line with a flexible cable in the field and they do break from time to time, especially if twisted together or around the cable.

On a slightly different note, crimping is an acceptable termination method under AS/NZS 2201.1. I believed for years that crimping was not a good termination method. I have in the last couple of years started crimping joins in security cables. I have found crimping to be an effective and efficient means of jointing as long as you have space for the join and it will not be exposed to the weather. I would however never crimp a resistor join as the solid conductor is too brittle. The standard backs this up by stating that only stranded cables are to be crimped. It also dictates that a ratchet style crimper must be used.

Ratchet Crimp Tool

Ratchet Crimp Tool

Pliers and cheap auto motive crimp tools do make an inferior connection.


Australian Standards – Security Systems – Part 1

August 17th, 2013

Recently I have had much discussion with industry work mates relating to the use, reference and implementation of the Australian standards relating to security systems. The standards that relate to installers are mainly:

AS NZS 2201.1-2007 Intruder alarm systems – Clients premises – Design installation commissioning and maintenance.
AS 2201.3-1991 Intruder alarm systems – Detection devices for internal use.

The vast majority of tech’s, sales people and suppliers I talk to have not looked at the actual standards in years if at all but are happy to paraphrase what they think is in the standards, often incorrectly.

To set the record straight Australian standards are not legislation and are not enforceable by law. That being said may contracts and specifications reference the standards and failure to comply in these cases may leave installers open to civil liability.
I have to admit that I paid little attention to the Australian standards for a long time. I just installed to a standard that I believed to be correct from general industry practices and experience. I now own copies of both the above standards and will discuss some myths and realities in the following posts.


ATMOD Now Available

November 25th, 2012

Jack Fuse is proud to announce the official release of the ATMOD.

The ATMOD is a pre-made end of line resistor pack with quick connect terminals that has been specifically designed for the security industry.

Once you start using the ATMOD you will never want to go back to resistors!

The ATMOD allows installers to terminate EOL resistors in a fraction of the time and eliminates mistakes or broken resistors. Standard alarm cable can be stripped and pushed into the ATMOD without the need for a screw driver, crimping or soldering. Even a brand new apprentice can’t get it wrong!

The ATMOD is available now from Seadan Security and Electronics in a range of DEOL values for common alarm/access panels. Click here for more technical information.

ATMOD EOL Resistor Pack

ATMOD EOL reed switch

ATMOD terminated to a reed switch


Fail Safe/Secure with PP8FR

August 25th, 2012

I have been asked how to power the locks for a fail safe/fail secure door using the PP8FR.

Fail safe/secure doors use two electric locks, one to release on fire or emergency and one to stay locked to maintain access control back into the building.

Often this is achieved by controlling the negative/ ground connection via the access control relay. The ground connection is fed from the power supply to the relay common and then the N/C and N/O relay contacts are fed, one to each lock so that when a card is presented the fail safe lock looses power and the fail secure lock turns on allowing access.

The problem comes when you need to fire trip the fail safe lock. The fail secure lock can’t be fire tripped as it will stop working. This is solved by breaking only the positive power feed to the fail safe lock. This can be easily done using separate fused outputs from the PP8FR. See diagram below.

Fail Safe/Secure with PP8FR

Fail Safe/Secure Door Set Up

The actual relay contacts used will vary depending on how the manufacturer labels their outputs. This method also has an added advantage, if one of the locks is tampered with and/or goes short circuit then only one fuse will blow leaving the other lock still functioning and keeping the door secure.