Landline telephone "ring trip"

[DIYspanner]

On the odd occasion when someone phones me, the phones don't ring but the incoming call light flashes on the phones.

If I pick up when the caller is still on, I can take the call. If I pickup after they put they've stopped calling, the line's dead and I have to press the plunger to get dial tone.

I'm read that this is a ring trip.

All 3 phones (2 wire) are new and I've rewired the extensions.

In the last week, I had Fire Optic broadband installed which means a new master socket plate has been installed.

I asked the BT engineer about this but he said it was a matter for the Openreach copper wire division of BT.


Are ring trips an external problem?

 

[bernardgreen]

They can be external.

They occur because something on the line is passing enough DC current to make the exchange believe the phone has been picked up.

ON hook the phone cannot pass DC, only AC via a capacitor to the bell or what ever makes the noise when the phone rings.

OFF hook the phone passes DC and the exchange detects this current and completes the call connection.

The external fault that causes this is damp and/or corroded joints where copper oxide has formed a diodic and resistive short on the line. This diode will convert the AC ringing voltage into enough DC current to ring trip the exchange.

Internal faults can be incorrectly wired extensions, ( terminals 2 and 5 reversed between sockets )

too many capacitors ( one in the master and others in two wire phones and micro filters )

sounders that have a diodic loading on the AC voltage ( together with incorrect wiring )

 

[DIYspanner]

Thanks for the reply posting bernardgreen.

Several years back when I was having trouble ADSL broadband speed, I bought a NTE5 master socket filtered lower faceplate to fit on the master socket.

I've been told that 2 wire phones don't actually need the bell filter but the BT engineer who came and installed the VDSL plate for the fibre-optic put the bell filter plate back on the VDSL plate.

The bell filter plate having a capacitor - a possible cause of ring tripping?

 

[cajar]

Disconnect the extensions at the master socket and test again. If it's still ring trip then it's something on BT's side. 90% of these are either faulty equipment or wet junction boxes / sockets in the house. Seldom external.

 

[AlanE]

If it was something external to the property how come the 'ring light' on the phone still functions? Is it not the same ring current that is powering this light?

 

[DIYspanner]

If it was something external to the property how come the 'ring light' on the phone still functions? Is it not the same ring current that is powering this light?

Not something I could comment on I'm afraid - I'm not an electronics expert but I certainly see your logic, thanks.

I have the original master lower plate without the bell filter (no capacitor). Maybe I should refit that and see if the ring trip still occurs.

Apart from an original extension cable that was plastered in to the wall over 20 years ago when the house was built, other extensions (2 daisy chained) and internal cables were installed by me and I know they are not damp or damaged.

 

[cajar]

The capacitor wont cause ring trip, it's probably low resistance when measured on one polarity but not the other, we call this a rectified loop. Usually caused by 'tracking' on a circuit board getting wet inside a socket or junction box, sometimes due to a faulty cable.

As a telephone engineer I would break the circuit down somewhere and test for low resistance, reversing the leads and testing again to confirm the fault, then break the line down further or remove each extension in turn until you find the cause. If you don't have a multimeter you can do what I said before - disconnect the faceplate and extensions and test for ring trip. Changing single components like the faceplate is a pretty laborious way to do it imo as it can be any number of things, and least likely to be the faceplate.

 

[bernardgreen]

The capacitor wont cause ring trip.

Some designs of loop detect circuit ( on the Line Card at the exchange } can be tripped by the AC current if there are too many ring capacitors and sounders on the line. Hence the maximum REN of 4 on most lines.

 

[cajar]

over 2 decades I have come across many fault reports of 'bell not ringing' caused by far too many ring capacitors (plus equipment capacitance). In these cases the line rings out but nothing is heard in the house. Not one of them was ring trip, which is where the call is 'answered' by the fault after one ring, but I'll take your word that it's possible.

 

[DIYspanner]

Many thanks for the postings folks, much appreciated.

May I ask if there's a device available for testing telephone extension cables with its phone connected?

What I mean is disconnecting an extension cable at the master socket and testing the cable with the extension phone plugged in.



Regarding the bell filter plate, I think I'll experiment with the original plate that doesn't have the filter.

 

[bernardgreen]

May I ask if there's a device available for testing telephone extension cables with its phone connected?

There might be but with a phone plugged in the results may not be accurate.

You can make up a tester with a plug and 3 resistors .

a 1 k Ω between wires 2 and 3

a 2.2 k Ω between wires 3 and 4

a 4.7 k Ω between wires 4 and 5

Plug that into one socket and then measure resistances between wires at the other sockets. If the measured resistances match the actual resistor values then the wiring is probably OK.

This of course means that all 4 wires ( 2 3 4 and 5 ) are connected which should happen anyway on an internal extension wiring,

 

[Astra99]

Bernard Green wrote: "This of course means that all 4 wires ( 2 3 4 and 5 ) are connected which should happen anyway on an internal extension wiring"

I wish I had a quid for every internal extension I have seem with only 2 and 5 connected!

Thus from your resistance values, with no connection on 3 or 4, the value between 2 and 5 should be (1 + 2.2 + 4.7) = 7.9kΩ. Yes?

 

[cajar]

May I ask if there's a device available for testing telephone extension cables with its phone connected?

There might be but with a phone plugged in the results may not be accurate.

You can make up a tester with a plug and 3 resistors .

a 1 k Ω between wires 2 and 3

a 2.2 k Ω between wires 3 and 4

a 4.7 k Ω between wires 4 and 5

Plug that into one socket and then measure resistances between wires at the other sockets. If the measured resistances match the actual resistor values then the wiring is probably OK.

This of course means that all 4 wires ( 2 3 4 and 5 ) are connected which should happen anyway on an internal extension wiring,

Pointless exercise, given the end user is looking for the cause of a ring trip fault which is usually down to LOW resistance.

OP, take your multimeter to the master socket and test towards your extensions (leave all equipment connected to the end of them). With extensions now disconnected from the master socket, test whichever wires were in 2 and 5 on the faceplate. You will set your multimeter to resistance, 2M Ohm range. Anything less than 1M Ohm is a fault. Switch polarity (switch leads around) and retest.

 

[cajar]

Bernard Green wrote: "This of course means that all 4 wires ( 2 3 4 and 5 ) are connected which should happen anyway on an internal extension wiring"

I wish I had a quid for every internal extension I have seem with only 2 and 5 connected!

Thus from your resistance values, with no connection on 3 or 4, the value between 2 and 5 should be (1 + 2.2 + 4.7) = 7.9kΩ. Yes?

yes, unless the extension socket has a ring capacitor in it, which means there is already a resistor between 2 and 3 too. A 470k ohm resister in parallel with your 1k ohm resistor between 2 and 3, will give you less than 1k ohm for that value and accordingly less than 7.9k ohm for 2 and 5. A lower than expected value is inconclusive, ie either caused by low insulation on the cable, or simply by the components in a normal socket, so will end with you opening the socket to test the wires themselves anyway.

 

[bernardgreen]

Cajar is right, the in service resistor ( 470 kΩ ) will affect the reading but the effect is less than the effect of the tolerance of the 1 kΩ.

The 470 kΩ in parallel with the 1 kΩ will read as 997.87 Ω (0.22% reduction )

It is quick test to ensure that cabling is correct. Insulation testing is a separate test sequence.