IE47562B1 - Protective circuits for electrically heated blankets or pads - Google Patents

Abstract

THE INVENTION RELATES TO A PROTECTIVE CIRCUIT COMPRISING AN INTERIOR DRIVER AND A OUTER MAIN CONNECT WITH A FUSE AND A FIRST RECTIFIER HALF-WAVE. [FR2408932A1]

Description

This invention relates to protective circuits for electrically heated blankets or pads.

British Patent Specification No. 1 155 118 discloses protective circuits for an electrically heated blanket or pad, each circuit comprising a dual coaxial heating element having inner and outer conductors, at least one of which is arranged to be connected across an AC supply, a fuse, and a half-wave rectifier means. The arrangement is such that in the event of a fault resulting in contact between the two conductors the half-wave rectifier means is bypassed so that the current through the fuse will change from halfwave rectified DC to AC whereby the current through the fuse will increase to cause the fuse to blow. While such circuits function, in general, satisfactorily, it has been found (as will be explained below) that it can be difficult to satisfactorily design them to ensure that in worst case tolerancing conditions the fuse will blow sufficiently quickly in the event of a fault.

According to the present invention a protective circuit for an electrically heated blanket or pad comprises a pair of input terminals for connection to an AC supply, a heating element having coaxially-arranged inner and outer conductors, a fuse, a first half-wave rectifier means, a series circuit connected between the input terminals and - 3 comprising said fuse, at least one of said conductors and the first half-wave rectifier means, the arrangement being such that in the event of a fault resulting in contact between the inner and outer conductors anywhere along the length of the conductors the first half-wave rectifier means will be bypassed so that the current flowing through the fuse will change from half-wave rectified DC to AC whereby the current through the fuse will increase to cause the fuse to blow, and a second half-wave rectifier means connected across said one conductor in such a direction that it does not normally conduct but in the event of a said fault it conducts during supply half-cycles of one polarity to further increase the current through the fuse.

The second half-wave rectifier means thus has the effect of increasing the fuse current in the event of a fault to speed up blowing of the fuse.

The invention will now be further described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a circuit diagram of a known protective circuit for an electrically heated blanket or pad; Figure 2 is a circuit diagram of a first protective circuit embodying the present invention; Figure 3 is a circuit diagram of a modification of the circuit shown in Figure 2; and Figure 4 is a circuit diagram of a further circuit embodying the present invention.

The protective circuit shown in Figure 1 is known and corresponds to that described with reference to Figure 2 of British Patent Specification No. 1 155 118. The circuit comprises a dual coaxial cable or heating element 1 comprising an inner conductor 2 and an outer conductor 3, both of which are resistance wires, shown for convenience as resistors. In practice, the inner conductor 2 will be wound helically on a suitable core of an electrically insulative material. The conductor 2 is then wrapped or sheathed with electrically insulative material, for instance a thermoplastic material such as polyvinyl chloride. The outer conductor 3 is wound helically over the layer of polyvinyl chloride and is then itself wrapped or sheathed with suitable electrically insulative material, which may again be polyvinyl chloride. The softening point of the polyvinyl chloride bewteen the conductors 2, 3 is about 150°C, which is well above the temperature that the heating element 1 reaches in normal use. However, it is not above the temperature which a portion of one of the conductors 2, 3 can reach in the event of a local overheating fault.

The heating element is connected as shown between a pair of input terminals 4 for connection to the live (L) and neutral (N) conductors of an AC supply (not shown).

A fuse 5 is connected in series with the inner conductor 2 and the adjacent terminal 4. (The fuse 5 could instead be connected between the outer conductor 3 and adjacent terminal 4). A half-wave rectifier means 6, which is preferably a semiconductor diode, is connected in series with and between the conductors 2, 3. More specifically, the end of the conductor 2 remote from the fuse 5 is connected through the rectifier 6 to the end of the conductor 3 which is nonad jacent thereto, the adjacent end of the conductor 3 being connected to the lower one of the terminals 4 as shown in the drawing.

In use, current flows through the conductors 2, 3, the conductors therefore generating heat. Due to the rectifier 6, the current is half-wave rectified DC.

However, if a high-temperature fault develops in the element 1 this may result in local heating which melts the material between the conductors 2, 3 sufficiently for the conductors to come into contact. When this happens, the rectifier 6 is short-circuited or bypassed so that nonrectified AC thereafter flows into the circuit and through the fuse 5. Consequently, the current through the fuse 5 is increased and it blows. The increase is due partly to the current increasing due to the absence of rectification and partly due to the fact that wherever the location of the fault the circuit resistance is halved (assuming the resistance of the conductors 2 and 3 to be the same).

The increase in the current will now be explained in more detail. In the .absence of a fault, the normal r.m.s. current IN is given by: 1 = v ·1 N 7Γ 2R where V = the r.m.s. mains input voltage? and R = the resistance of each conductor 2, 3.

In the event of a fault resulting in contact between the two conductors 2, 3, then regardless of the position of the fault the nominal fault current I is given by: I = V FN 5 .

Therefore, the ratio IpN/IN of nominal fault current to normal current is given by: Ι__τ = V . 2/Sr = 2/Γ = 2.8: 1.

FN — R V · ΧΝ The ratio 1-,,/1,. is therefore such as to lead one FN N to suppose that in all circumstances the fuse will blow quickly in the event of a fault. However, as will now be explained, practical considerations may dictate otherwise. Suppose that the tolerance on the input voltage V is +6°:/o and -10% and that the tolerance on the resistance R is ± 8°Z. In this case, the minimum value of the fault current {IFmin) is given by: ^•Fmin — TFN = 0.83 I 1.08 FN (I.

Nmax) Similarly, the maximun value of the normal current is given by: I,T = 1.06 I„ = 1.15 I„ Nmax - Ν N 0.92 One would therefore choose a fuse rating near to 1.15 Ijj, whereby the fuse should then blow if the current increases by the ratio: I„ . = 0.83 x 2.8 2 : 1 Puna ^Nmax 1.15 In practice, however, the fuse rating may need to be the nearest preferred, standard value above 1.15 IN, which may mean that the fault current is less than twice the fuse rating. If, for example, one employed a British Standard approved 20 mm quick-blow glass cartridge fuse, the blowing time in the worst case could be as long as 30 minutes, which is, of course, an unacceptably long time, in that considerable overheating could occur before the fuse blows.

A disadvantage of the circuit of Figure 1 is that both the conductors 2, 3 carry the load current so that a break in the outer conductor 3 giving rise to an arc could create a fire hazard even though the fuse might eventually blow due to the arc causing contact with the inner conductor. Another disadvantage is that short circuit failure of the diode 6 leads to greatly increased heating. These disadvantages could be obviated by employing the circuit shown in Figure 3 of British Patent Specification No. 1 155 118 in which the outer conductor does not normally carry current. However, the circuit of Figure 3 of British Patent Specification No. 1 155 118 has the disadvantage that for faults in some positions there is no or little reduction in circuit resistance in the event of a fault, the only increase being due to the cessation of rectification, whereby of the circuits disclosed in British Patent Specification No. 1 155 118 that described above with reference to Figure 1 hereof is preferred.

Figure 2 shows a protective circuit in accordance with the invention. The circuit of Figure 2 is in many respects similar to that shown in Figure 1 and will only be described in so far as it differs. In the case of Figure 2 the outer conductor 3 does not carry the load current. Preferably, it is a low resistance copper conductor rather than a resistance heating conductor, whereby it is represented by a thick line rather than a resistor. One end of the conductor 3 is connected to the side of the rectifier 6 remote from the conductor 2, and the other end of the conductor 3 is floating.

A second half-wave rectifier means 7, preferably a semiconductor diode, is connected across the conductor 2 with the polarity shown whereby in normal use it does not conduct current.

What happens when a fault resulting in contact between the conductors 2, 3 will new be described. If the fault occurs at the left hand end of the heating element 1, there will be a virtual shortcircuit between the terminals 4 whereby the fault current will be very large and will blow the fuse 5 instantly. If the fault occurs at the right hand end of the element 1, the second rectifier 7 is connected across the terminals 4 with virtually no limiting resistance, whereby again the fault current will be very large and will blow the fuse 5 instantly. Clearly, the fault current will approach a minimun value as the location of the fault approaches the centre of the heating element 1. In the event of a fault halfway along the element 1, the current is increased due to the cessation of rectification, due to a reduction in circuit resistance, and due to the rectifier 7. In positive half-cycles, the circuit resistance is reduced from 2R to R since current flows from live to neutral via only the left hand half of the conductor 2. In negative half cycles current will flow, due to bypassing of the rectifier 6, and the circuit resistance is in this case reduced to R/2 since current flows from neutral to live through both halves of the conductor 2 in parallel, that is to say directly through the left hand half and, via the rectifier 7, through the right hand half.

If the resistance of the whole conductor 2 is 2R, as before the normal current is given by: I = ' V N —— 2/Γ R For a fault at the centre of the conductor 2 the nominal fault current I is given by: I™ 2 + V 2 - V FN /Tr + /Tit · 2 I 21., 2 - 41 1 . 1 Nl Nl 2 .

Accordingly, Ι_.τ = /2Z + . ’'’Ν' which is approximately equal to 4.5Ijj. That is to say, the nominal fusing current is now 4,5 times the normal current.

Applying the tolerancing of voltage and resistance as set forth above in the description of Figure 1, the absolute worst case fault current is given by: 0.83 x 4.5 = 3.2 I . 1.15 Assuming that the same type of fuse is used as in the circuit of Figure 1, a fault current of only 2.75 times the fuse rating would reduce the blowing time to an absolute worst case maximum of only 2 seconds, which of course compares very favourably indeed to the figure of 30 minutes for Figure 1.

The circuit of Figure 2 is also advantageous over that of Figure 1 in the following respects. A short-circuit failure of either of the rectifiers 6 and 7 causes, the fuse to blow. The cost of the element 1 is reduced since the outer conductor 3 can be of copper of a low resistivity alloy rather than a relatively expensive nickel alloy as used for the inner, heating conductor 2. Since the outer conductor 3 carries no current during normal use, a break therein will not give rise to arcing. Further, protection is also afforded against arcing resulting from a break in the inner conductor 2, since it is highly improbable that such an arc could burn its way right out of the cable in the time taken for the fuse to blow.

The circuit described in Figure 2 can be modified as shovzn in Figure 3 by connecting together the ends of the outer conductor 3 outside of the element 1 by a conductor 8, whereby should a break occur in the outer conductor the protective facility will not thereby be impaired, that is to say a circuit bypassing the rectifier 6 will be established in the event of an overheating fault even if there is a break in the conductor 3.

Figure 4 shows another protective circuit embodying 10 the invention. The circuit of Figure 4 resembles that of Figure 2 in many respects and will only be described in so far as it differs therefrom. Xn the circuit, a second heating element 1' is provided, the element 1' being similar to the element 1 and comprising inner and outer conductors 2' and 3' similar to the conductors 2 and 3 of the element 1. A third rectifier 7' is connected across the conductor 2' in the same way as the second rectifier 7 is connected across the conductor 2.

The circuit of Figure 4 operates similarly to the 20 circuit of Figure 2 and has the same advantageous features as set forth above for the circuit of Figure 2. It has, however, the additional advantage that an even higher fault current is obtained.

As for the circuit of Figure 2, in the circuit of 25 Figure 4 a virtual short-circuit across the supply will be established if, due to a fault, contact occurs between the conductors of either of the elements 1, 1’ at either of their ends; and the minimum fault current will arise in the case of such a fault occurring half way along either element.

If the resistance of each of the conductors 2, 2' 47568 is R, then as before the normal current is given by: IN = V 2/Γ. R.

Consider what happens if a fault occurs half way along the element 1'. The current flows will be the same as in Figure 3, except that the individual components will be doubled since the resistance of the conductor 2’ is halved. Consequently, the nominal fault current is given by: 1 2 F +2V 2 -2V . 2 2 + 4I„ 1 2 -81 1 2 • 1 Ν , + η Accordingly, = /42 + 82 .IM, which is N approximately equal to 9IN· That is to say, the nominal fusing current is now nine times the normal current.

Applying the tolerancing of voltage and resistance as set forth above in the description of Figure 1, the absolute worst case fault current is given by: 0.83 x 9 = 6.5 I. 1.15 which will reduce the maximum absolute worst case fuse blowing time to a few milliseconds or so.

The circuit of Figure 4 can be modified by connecting together the ends of at least one of the outer 2o conductors 3, 3' in the manner explained above with reference to Figure 3 and for the same reason.

The invention can be embodied in other ways than those described above by way of example. For instance, the invention can be applied to circuits in which both of the conductors 2 and 3 are heating conductors. As an example 7 5 6 2 of this, the circuit of Figure 1 hereof could be modified to· constitute a circuit in accordance with the invention by connecting a rectifier 7 across the conductor 2 in the same manner as in Figures 2 to 4 hereof. The circuit of Figure 1 of British Patent Specification No. 1 155 118 which is the same as that of Figure 1 hereof except that the left hand and right hand ends of the conductor 3 are connected to the neutral terminal 4 and the cathode of the diode 6, respectively, could be modified in the same manner to constitute a circuit in accordance with the invention.

Although such modified circuits would not possess all of the advantageous features of the embodiments of Figures 2 and 3, they would in particular have the advantage that the fault current would be increased by virtue of provision of the second rectifier 7.

Although the first rectifier 6 in each of the above embodiments is preferably a semiconductor diode, it can in each case be replaced by any other half wave rectifier means, in particular a thyristor. If a thyristor is employed, there is preferably also provided a control circuit to control firing of the thyristor in such a manner as to control the current flowing through the heating element or elements.

In this way, the thyristor serves a second function of controlling the heat output of the blanket or pad.

Claims (11)

CLAIMS:

1. A protective circuit for an electrically heated blanket or pad, comprising a pair of input terminals for connection to an AC supply, a heating element having 5 coaxially-arranged inner and outer conductors, a fuse , a first half-wave rectifier means, a series circuit connected between the input terminals and comprising said fuse, at least one of said conductors and the first half-wave rectifier means, the arrangement being such that in the 10 event of a fault resulting in contact between the inner and outer conductors anywhere along the length of the conductors the first half-wave rectifier means will be bypassed so that the current flowing through the fuse wll change from halfwave rectified DC to AC whereby the current through the fuse 15 will increase to cause the fuse to blow, and a second halfwave rectifier means connected across said one conductor in such a direction that it does not normally conduct but in the event of a said fault it conducts during supply halfcycles of one polarity to further increase the current 2. Θ through the fuse.

2. A protective circuit according to claim 1, wherein said other conductor is so connected that it does not conduct current in the absence of a said fault. 47S62

3. A protective circuit according to claim 2, wherein one end of said other conductor is connected to said series circuit on the side of the first half-wave rectifier means remote fran said cne conductor and the other end of said other 5 conductor is floating.

4. A protective circuit according to claim 2, wherein one end of said other conductor is connected to said series circuit on the side of the first half-wave rectifier means remote from said one conductor and the two ends of said 10 other conductor are connected together.

5. A protective circuit according to any one of the preceding claims, wherein said one conductor is a heating conductor and said other conductor is of low resistance relative to the heating conductor. 15

6. A protective circuit according to any one of claims 1 to 5, wherein the first half-wave rectifier means is a diode.

7. A protective circuit according to any one of claims 1 to 5, wherein the first half-wave rectifier means is a 20 thyristor and a control circuit is provided to control firing of the thyristor in such a manner as to control the current flowing through the heating element to control the heat output of the blanket or pad.

8. A protective circuit according to any one of the 25 preceding claims, wherein said one conductor is the inner conductor and said other conductor is the outer conductor.

9. A protective circuit according to any one of the preceding claims, comprising a second heating element having coaxially arranged inner and outer conductors of which at least one comprises part of said series circuit, the arrangement being such that in the event of a fault in either heating element resulting in contact between the inner and outer conductors anywhere along the length of the conductors the first half-wave rectifier means will be bypassed so that the current flowing through the fuse will change from halfwave rectified DC to AC whereby the current through the fuse will increase to cause the fuse to blow, and a third halfwave rectifier means connected across said one conductor of the second heating element in such a direction that it does not normally conduct but in the event of a said fault at least in the second heating element it conducts during supply half cycles of one polarity to further increase the current through the fuse.

10. A protective circuit according to claim 9, wherein said one conductor of the second heating element, said one conductor of the first mentioned heating element and the first half-wave rectifier means are connected in series in that order.

11. A protective circuit for an electrically heated blanket or pad substantially as herein described with reference to any one of Figures 2 to 4 of the accompanying drawing.